$string = "5eviltext";
$count = strlen($string);
$charPool = array_merge(range('a','z'), range('A', 'Z'), range('0', '9'));
for ( $i = 0; $i < $count; $i++ ) {
	if ( in_array($string[$i], $charPool) != false ) {
		echo $string[$i]."\r\n";
		$cleanstring .= $string[$i];
	} else {
		echo 'invalid '.$string[$i]."\r\n";
	}
}