In response to 'ibolmo', this is an extended version of string_walk, allowing to pass userdata (like array_walk) and to have the function edit the string in the same manner as array_walk allows, note now though that you have to pass a variable, since PHP cannot pass string literals by reference (logically).
<?php
function string_walk(&$string, $funcname, $userdata = null) {
for($i = 0; $i < strlen($string); $i++) {
$hack = $string{$i};
call_user_func($funcname, &$hack, $i, $userdata);
$string{$i} = $hack;
}
}
function yourFunc($value, $position) {
echo $value . ' ';
}
function yourOtherFunc(&$value, $position) {
$value = str_rot13($value);
}
string_walk($x = 'interesting', 'yourFunc');
string_walk($x = 'interesting', 'yourOtherFunc');
echo $x;
?>
Also note that calling str_rot13() directly on $x would be much faster ;-) just a sample.