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.

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.

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, ';
foo2( $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, ';
foo2( $str );
echo $str;    // outputs 'This is a string, '
foo2( &$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.
      

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.