filter allowed chars using PHP

function allowed_chars($text){
    $allowed = array(
        "1","2","3","4","5","6","7","8","9","0",
        "a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z",
        "A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z",
        "ا","أ","إ","آ","ض","ص","ث","ق","ف","غ","ع","ه","خ","ح","ج","د","ش","س","ي","ب","ل","ت","ن","م","ك","ط","ئ","ء","ؤ","ر","ى","ة","و","ز","ظ","ذ"
        ," ","-","_"
    );
    $splitted = str_split_utf8($text);
    $text = "";
    foreach($splitted as $letter){
        if(in_array($letter,$allowed)){
            $text .= $letter;
        }
    }
    return $text;
}


function str_split_utf8($str) {
    // place each character of the string into array
    $split=1;
    $array = array();
    for ( $i=0; $i < strlen( $str ); ){
        $value = ord($str[$i]);
        if($value > 127){
            if($value >= 192 && $value <= 223)
                $split=2;
            elseif($value >= 224 && $value <= 239)
                $split=3;
            elseif($value >= 240 && $value <= 247)
                $split=4;
        }else{
            $split=1;
        }
        $key = NULL;
        for ( $j = 0; $j < $split; $j++, $i++ ) {
            $key .= $str[$i];
        }
        array_push( $array, $key );
    }
    return $array;
}


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