PHP: Loop through each character in a string

function.php

<?php
$str = "String to loop through"
$strlen = strlen( $str );
for( $i = 0; $i <= $strlen; $i++ ) {
    $char = substr( $str, $i, 1 );
    // $char contains the current character, so do your processing here
}
?>

use_case.php

<?php
$str = "123?param=value"
$strlen = strlen( $str );
$id = "";
for( $i = 0; $i <= $strlen; $i++ ) {
    $char = substr( $str, $i, 1 );
    if( ! is_numeric( $char ) ) { break; }
    $id .= $char;
}
// $id now contains the ID I need, in this case: 123
?>