FUNCTION

A function may be defined using syntax such as the following:

function foo ($arg_1, $arg_2, ..., $arg_n) {
    echo "Example function.\n";
    return $retval;
}
     

Any valid PHP3 code may appear inside a function, even other functions and class definitions.

Functions must be defined before they are referenced.

Returning values

Values are returned by using the optional return statement. Any type may be returned, including lists and objects.

function my_sqrt ($num) {
    return $num * $num;
}
echo my_sqrt (4);   // outputs '16'.
      

Multiple values may not be returned, but the same effect can be achieved by returning a list:

function foo() {
   return array (0, 1, 2);
}
list ($zero, $one, $two) = foo();
      

Arguments

Information may be passed to functions via the argument list, which is a comma-delimited list of variables and/or constants.

PHP3 supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists are not supported, but a similar effect may be achieved by passing arrays.

function takes_array($input) {
    echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
      

Passing by reference

By default, function arguments are passed by value. If you wish to allow a function to modify its arguments, you may pass them by reference.

If you wish a function's argument to always be passed by reference, you can prepend an ampersand (&) to the argument name in the function definition:

function foo( &$bar ) {
    $bar .= ' and something extra.';
}
$str = 'This is a string, ';
foo ($str);
echo $str;    // outputs 'This is a string, and something extra.'
       

If you wish to pass a variable by reference to a function which does not do this by default, you may prepend an ampersand to the argument name in the function call:

function foo ($bar) {
    $bar .= ' and something extra.';
}
$str = 'This is a string, ';
foo ($str);
echo $str;    // outputs 'This is a string, '
foo (&$str);
echo $str;    // outputs 'This is a string, and something extra.'
       

Default values

A function may define C++-style default values for scalar arguments as follows:

function makecoffee ($type = "cappucino") {
    echo "Making a cup of $type.\n";
}
echo makecoffee ();
echo makecoffee ("espresso");
       

The output from the above snippet is:

Making a cup of cappucino.
Making a cup of espresso.
      

The default value must be a constant expression, not (for example) a variable or class member.

Note that when using default arguments, any defaults should be on the right side of any non-default arguments; otherwise, things will not work as expected. Consider the following code snippet:

function makeyogurt ($type = "acidophilus", $flavour) {
    return "Making a bowl of $type $flavour.\n";
}

echo makeyogurt ("raspberry");   // won't work as expected
       

The output of the above example is:

Warning: Missing argument 2 in call to makeyogurt() in 
/usr/local/etc/httpd/htdocs/php3test/functest.html on line 41
Making a bowl of raspberry .
      

Now, compare the above with this:

function makeyogurt ($flavour, $type = "acidophilus") {
    return "Making a bowl of $type $flavour.\n";
}

echo makeyogurt ("raspberry");   // works as expected
       

The output of this example is:

Making a bowl of acidophilus raspberry.