Filtrado de caracteres Mediante ASCII [PHP]

<?php 
function detectar($ascii) { 
for ($i = 0; $i < strlen($ascii); $i++) { 
$byte = ord($ascii{$i}); 
if($byte >=48 && $byte <=57 || $byte >=64 && $byte <=90 || $byte >=97 && $byte <=122) { 
$hex .= chr($byte); 
} else { print "Injeccion de Caracteres Detectado!\n"; exit; } 
} 
return $hex; 
} 
 
foreach($_GET as $var => $value) { 
$value = detectar($value); 
$_GET[$var] = $value; 
} 
 
foreach($_POST as $var => $value) { 
$value = detectar($value); 
$_GET[$var] = $value; 
} 
?>