User Defined Functions

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

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.'

function foo() {
  echo "In foo()";
}

function bar( $arg = '') {
  echo "In bar(); argument was '$arg'";
}

$func = 'foo';
$func();
$func = 'bar';
$func( 'test' );