<?php
if(isset($_POST['submit'])){
    $alan = explode(" ", $_POST['alan']);
    $filtre = ['lorem','ipsum','dolor'];
    $diff = array_diff($alan,$filtre);
    if (empty($diff)) {
        $out = '<script>alert("'.implode(', ',$filtre).' kelimeleri kullanılamaz.'.'");</script>';
    }else{
        //çalıştır
    }
}
?>

<form action="" method="post">
<div class="form-group">
  <div class="col-sm-12">
    <input name="alan" type="text" class="form-control" placeholder="yazi" required>
    <?php if(isset($out)){echo $out;} ?>
    <input type="submit" name="submit">
  </div>
</div>
</form>