CGS 3066: Introduction to Internet Technology
Spring 2003
Lecture #14: Writing PHP Functions

Outline

Simple Functions

One of the important features of programming in general is the concept of functions
We have already learned how to call functions, and seen how useful they are (calculating various math functions, printing out text, getting the time of day, ...)
The main use of functions is to store a sequence of code that is used many times. Functions help to increase readability as well.
Sometimes functions return a value (like in the case of the date() function, and sometimes they just handle a sequence of instructions and don't return a result. These types that don't return results are sometimes called procedures.

We can create a function as follows:

function /* Function name */ ( /* Arguments of function, separated by commas */ )
{
    /* Code for function goes here */
}

The arguments for a function are specified just by a variable name, and are separated by commas. Thus it is not like C/C++/Java, where function arguments need to specify the variable type as well.
It is possible to name the arguments to a function with the same names as other variables, because all the function uses the argument names for is placeholders for the values of the variables.
You can never omit the curly braces ({}) for a function, like you can with if-statements and such. They are absolutely always required.
If you want to return a value from a function (say a sum), then you use the keyword return followed by the variable or expression to return the value of.

Here is sample code for some basic PHP functions:

Simple PHP Functions (samp14a.php)
<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Sample PHP Page 14A</title> <link rel="stylesheet" type="text/css" href="../styles.css" /> </head> <body> <p> <?php // Writing a simple function // Sum two parameters and return the total function sum($a, $b) { $c = $a + $b; return $c; } // Sum an entire array of values (a list) function sum_array($sum_arr) { $total = 0; foreach ($sum_arr as $x) $total += $x; return $total; } // Print the contents of an array of values (a list) function print_array($arr) { print( "Array: " ); foreach ($arr as $x) print( "$x " ); print( "<br />\n" ); } $x = 3; $y = 4; $my_array = array(1, 2, 3, 4, 5); // Calling functions with variables $result = sum($x, $y); print( "Sum: $x+$y=$result<br />\n" ); $arr_result = sum_array($my_array); print_array($my_array); print( "Array Sum: $arr_result<br />\n" ); // Calling functions with variables and/or values $result = sum($x, 10); print( "Sum: $x+10=$result<br />\n" ); $arr_result = sum_array(array(2,4,6,8)); print_array(array(2,4,6,8)); print( "Array Sum: $arr_result<br />\n" ); ?> </p> <!-- Validation Icon and Information for page --> <p> <a href="http://validator.w3.org/check/referer"><img src="../valid-xhtml10.png" alt="Valid XHTML 1.0!" height="31" width="88" /></a> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img src="../vcss.png" alt="Valid CSS!" height="31" width="88" /></a> </p> </body> </html>

Simple PHP Functions (samp14a.php)

<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Sample PHP Page 14A</title>
    <link rel="stylesheet" type="text/css" href="../styles.css" />
  </head>
  <body>

    <p>
      <?php
        // Writing a simple function

          // Sum two parameters and return the total
        function sum($a, $b)
        {
            $c = $a + $b;
            return $c;
        }

        // Sum an entire array of values (a list)
        function sum_array($sum_arr)
        {
            $total = 0;
            foreach ($sum_arr as $x)
                $total += $x;
            return $total;
        }

        // Print the contents of an array of values (a list)
        function print_array($arr)
        {
            print( "Array: " );
            foreach ($arr as $x)
                print( "$x " );
            print( "<br />\n" );
        }

        $x = 3;
        $y = 4;
        $my_array = array(1, 2, 3, 4, 5);

        // Calling functions with variables
        $result = sum($x, $y);
        print( "Sum: $x+$y=$result<br />\n" );

        $arr_result = sum_array($my_array);
        print_array($my_array);
        print( "Array Sum: $arr_result<br />\n" );

        // Calling functions with variables and/or values
        $result = sum($x, 10);
        print( "Sum: $x+10=$result<br />\n" );

        $arr_result = sum_array(array(2,4,6,8));
        print_array(array(2,4,6,8));
        print( "Array Sum: $arr_result<br />\n" );

      ?>
    </p>

    <!-- Validation Icon and Information for page -->
    <p>
      <a href="http://validator.w3.org/check/referer"><img
      src="../valid-xhtml10.png"
      alt="Valid XHTML 1.0!" height="31"
      width="88" /></a>
      <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
      src="../vcss.png"
      alt="Valid CSS!" height="31"
      width="88" /></a>
    </p>
  </body>
</html>

Dealing with Globals

So far we have been dealing simply with variables as parameters to functions. What if we want to pass fewer parameters to functions and use global variables instead?
In C/C++/Java, globals are always easily accessed and do not require any explicit handling routines. However in PHP, the scoping of variables is quite different.
Local variables are those that are inside of functions (and not in parameters or global). They only exist for as long as the code in the function is executing. Afterwards they do not retain any value.
Global variables are mostly what we used in the previous lectures. These are the variables that are just specified in the regular php section of the code. These variables exist until the php file has been sent to the user's web browser.
In order to see global variables in a function, we must use the keyword global followed by each of the global variables we want to see, separated by commas.

Here is sample code for dealing with globals in PHP:

Globals and Functions in PHP (samp14b.php)
<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Sample PHP Page 14B</title> <link rel="stylesheet" type="text/css" href="../styles.css" /> </head> <body> <p> <?php // Declare some global variables $g1 = 4; $g2 = 3; // Sum and return the total of $a, $b, $g1, and $g2 // Notice that only $a and $b are summed, since // integers default to 0 if they don't exist function sum($a, $b) { $c = $a + $b + $g1 + $g2; print( "inner c: $c<br />\n" ); return $c; } // Sum and return the total of $a, $b, $g1, and $g2 // This time use the global keyword function sum_dwg($a, $b) { global $g1, $g2; $c = $a + $b + $g1 + $g2; print( "inner c: $c<br />\n" ); return $c; } // Sum and return the total of $a, $b, $g1, and $g2 // This time use the global keyword // Also declare the helper $c to be global, so it will // stick around after the function is done function sum_special($a, $b) { global $g1, $g2, $c; $c = $a + $b + $g1 + $g2; print( "inner c: $c<br />\n" ); return $c; } // Print each member of the superglobal $GLOBALS function print_sg() { print( "All Global Variables:<br />\n" ); foreach ($GLOBALS as $id => $value) print( "$id => $value<br />\n" ); } print( "Global Vars:<br />\ng1: $g1<br />\ng2: $g2<br /><br />\n" ); $x = 11; $y = 20; // Calling functions without explicit global $result = sum($x, $y); // c is a local variable to sum(), so it has fallen out of scope // so this should print nothing print( "outer c = $c<br />\n" ); print( "sum($x,$y) = $result<br /><br />\n" ); // Calling functions with explicit global $result = sum_dwg($x, $y); // c is a local variable to sum_dwg(), so it has fallen out of scope // so this should print nothing print( "outer c = $c<br />\n" ); print( "sum_dwg($x,$y) = $result<br /><br />\n" ); // Calling functions with explicit global and global c $result = sum_special($x, $y); // c is specified global to sum_special(), so it is in scope print( "outer c = $c<br />\n" ); print( "sum_special($x,$y) = $result<br /><br />\n" ); // Print the superglobal $GLOBALS print_sg(); ?> </p> <!-- Validation Icon and Information for page --> <p> <a href="http://validator.w3.org/check/referer"><img src="../valid-xhtml10.png" alt="Valid XHTML 1.0!" height="31" width="88" /></a> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img src="../vcss.png" alt="Valid CSS!" height="31" width="88" /></a> </p> </body> </html>

Globals and Functions in PHP (samp14b.php)

<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Sample PHP Page 14B</title>
    <link rel="stylesheet" type="text/css" href="../styles.css" />
  </head>
  <body>

    <p>
      <?php
        // Declare some global variables
        $g1 = 4;
        $g2 = 3;

        // Sum and return the total of $a, $b, $g1, and $g2
        // Notice that only $a and $b are summed, since 
        // integers default to 0 if they don't exist
        function sum($a, $b)
        {
            $c = $a + $b + $g1 + $g2;
            print( "inner c: $c<br />\n" );
            return $c;
        }

        // Sum and return the total of $a, $b, $g1, and $g2
        // This time use the global keyword
        function sum_dwg($a, $b)
        {
            global $g1, $g2;
            $c = $a + $b + $g1 + $g2;
            print( "inner c: $c<br />\n" );
            return $c;
        }

        // Sum and return the total of $a, $b, $g1, and $g2
        // This time use the global keyword
        // Also declare the helper $c to be global, so it will 
        // stick around after the function is done
        function sum_special($a, $b)
        {
            global $g1, $g2, $c;
            $c = $a + $b + $g1 + $g2;
            print( "inner c: $c<br />\n" );
            return $c;
        }

        // Print each member of the superglobal $GLOBALS
        function print_sg()
        {
            print( "All Global Variables:<br />\n" );
            foreach ($GLOBALS as $id => $value)
                print( "$id => $value<br />\n" );
        }

        print( "Global Vars:<br />\ng1: $g1<br />\ng2: $g2<br /><br />\n" );

        $x = 11;
        $y = 20;

        // Calling functions without explicit global
        $result = sum($x, $y);
        // c is a local variable to sum(), so it has fallen out of scope
        // so this should print nothing
        print( "outer c = $c<br />\n" );
        print( "sum($x,$y) = $result<br /><br />\n" );

        // Calling functions with explicit global
        $result = sum_dwg($x, $y);
        // c is a local variable to sum_dwg(), so it has fallen out of scope
        // so this should print nothing
        print( "outer c = $c<br />\n" );
        print( "sum_dwg($x,$y) = $result<br /><br />\n" );

        // Calling functions with explicit global and global c
        $result = sum_special($x, $y);
        // c is specified global to sum_special(), so it is in scope
        print( "outer c = $c<br />\n" );
        print( "sum_special($x,$y) = $result<br /><br />\n" );

        // Print the superglobal $GLOBALS
        print_sg();

      ?>
    </p>

    <!-- Validation Icon and Information for page -->
    <p>
      <a href="http://validator.w3.org/check/referer"><img
      src="../valid-xhtml10.png"
      alt="Valid XHTML 1.0!" height="31"
      width="88" /></a>
      <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
      src="../vcss.png"
      alt="Valid CSS!" height="31"
      width="88" /></a>
    </p>
  </body>
</html>

There are some variables called superglobals since they are always accessible. An example is $GLOBALS. This is an array containing all of the global variables that currently exist. It is indexed by the variable name (minus the $).
In the example, notice that there are many other global variables we didn't even know existed. Also notice that $x, $y, $c and $result are also included since they are global at that point too.
There are several other superglobals, but this one is one of the more useful, since we can use it to access globals without declaring them global ahead of time in a function.

Static Variables

Whenever we deal with functions, we usually have some local variables. Sometimes we would like to have some of those values remember their previous state without having to use a global variable.
Normally a local variable only has its value while the function is running (while it is in scope). Sometimes these are called dynamic variables. Static variables retain their value, much like globals, but are specific to the function.
We can declare a variable as static by using the keyword static followed by the variable's normal declaration. It is even possible to initialize static variables to a starting value with this syntax.

Here is sample code showing the difference between static and dynamic variables in PHP:

PHP Static Variables (samp14c.php)
<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Sample PHP Page 14C</title> <link rel="stylesheet" type="text/css" href="../styles.css" /> </head> <body> <p> <?php // Make a simple function that increments a local variable function simple() { $a = 0; $a++; print( "a: $a<br />\n" ); } // Make a simple function that increments a static local variable function simple_use_static() { static $a = 0; $a++; print( "a: $a<br />\n" ); } // Call the simple function 5 times for ($i = 0; $i < 5; $i++) simple(); print( "<br />\n" ); // Call the simple_use_static function 5 times for ($i = 0; $i < 5; $i++) simple_use_static(); ?> </p> <!-- Validation Icon and Information for page --> <p> <a href="http://validator.w3.org/check/referer"><img src="../valid-xhtml10.png" alt="Valid XHTML 1.0!" height="31" width="88" /></a> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img src="../vcss.png" alt="Valid CSS!" height="31" width="88" /></a> </p> </body> </html>

PHP Static Variables (samp14c.php)

<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Sample PHP Page 14C</title>
    <link rel="stylesheet" type="text/css" href="../styles.css" />
  </head>
  <body>

    <p>
      <?php
        // Make a simple function that increments a local variable
        function simple()
        {
            $a = 0;
            $a++;
            print( "a: $a<br />\n" );
        }

        // Make a simple function that increments a static local variable
        function simple_use_static()
        {
            static $a = 0;
            $a++;
            print( "a: $a<br />\n" );
        }

        // Call the simple function 5 times
        for ($i = 0; $i < 5; $i++)
            simple();

        print( "<br />\n" );

        // Call the simple_use_static function 5 times
        for ($i = 0; $i < 5; $i++)
            simple_use_static();
      ?>
    </p>

    <!-- Validation Icon and Information for page -->
    <p>
      <a href="http://validator.w3.org/check/referer"><img
      src="../valid-xhtml10.png"
      alt="Valid XHTML 1.0!" height="31"
      width="88" /></a>
      <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
      src="../vcss.png"
      alt="Valid CSS!" height="31"
      width="88" /></a>
    </p>
  </body>
</html>

Pass by Reference

Normally function parameters are passed by value. This means that any modifications to their value in the function do not propagate back out to the caller of the function.
It is possible to have function parameters passed by reference. This means that changes to that variable in the function will be propagated back to the caller of the function.
This is another way to circumvent the need for using global variable references, but it is sometimes confusing.
If a variable is to be passed by reference, we prefix its name in the function argument list with an ampersand (&) before the $.
Once again, this has some differences than C/C++. In these languages, arrays were always automatically passed by reference. In PHP, all variables (including arrays) are passed by value as the default. So you need to explicitly do a pass by reference for arrays as well.

Here is sample code showing the difference between pass by value and pass by reference in PHP:

PHP Pass by Reference (samp14d.php)
<?php print("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"); ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Sample PHP Page 14D</title> <link rel="stylesheet" type="text/css" href="../styles.css" /> </head> <body> <p> <?php // Simple pass by value function function pbv($x) { $x++; print( "x in function: $x<br />\n" ); } // Simple pass by reference function function pbr(&$x) { $x++; print( "x in function: $x<br />\n" ); } // Simple array pass by value function function arr_pbv($arr) { $arr[0]++; print( "arr[0] in function: $arr[0]<br />\n" ); } // Simple array pass by reference function function arr_pbr(&$arr) { $arr[0]++; print( "arr[0] in function: $arr[0]<br />\n" ); } $x = 3; // Demonstrating pass by value print( "x before function: $x<br />\n" ); pbv($x); print( "x outside function: $x<br />\n" ); print( "<br />\n" ); // Demonstrating pass by reference print( "x before function: $x<br />\n" ); pbr($x); print( "x outside function: $x<br />\n" ); print( "<br /><br />\n" ); $arr = array(4, 5, 6); // Demonstrating pass by value print( "arr[0] before function: $arr[0]<br />\n" ); arr_pbv($arr); print( "arr[0] outside function: $arr[0]<br />\n" ); print( "<br />\n" ); // Demonstrating pass by reference print( "arr[0] before function: $arr[0]<br />\n" ); arr_pbr($arr); print( "arr[0] outside function: $arr[0]<br />\n" ); ?> </p> <!-- Validation Icon and Information for page --> <p> <a href="http://validator.w3.org/check/referer"><img src="../valid-xhtml10.png" alt="Valid XHTML 1.0!" height="31" width="88" /></a> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img src="../vcss.png" alt="Valid CSS!" height="31" width="88" /></a> </p> </body> </html>

PHP Pass by Reference (samp14d.php)