Anti-Spam Basic PHP

$string = str_replace(" ","",$post);
// $string = str_replace("spam","",$string);
$string = str_replace("
","",$string);
$string = str_replace("\n","",$string);
$string = str_replace("idiot","",$string);
$string = str_replace("stupid","",$string);
$string = str_replace("nom","",$string);
$string = str_replace("pam","",$string);
$string = str_replace("nom","",$string);
$string = str_replace("dumb","",$string);
$string = str_replace("crap","",$string);
$string = str_replace("dick","",$string);
$string = str_replace("balls","",$string);
$string = str_replace("gay","",$string);
$string = preg_replace("/[^a-zA-Z0-9\s]/", "", $string);
 
$string = strtolower($string);
 
$pos = strpos($string,"spam");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"retard");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"fat");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"ass");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"shit");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"fuck");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"df");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"fk");
if($pos === false){ $string = $string; }
else { $string = ""; }
 
$pos = strpos($string,"asdf");
if($pos === false){ $string = $string; }
else { $string = ""; }