checking is specific words are in the form

[seany123]

im currently in the progress of doing a page where players can have avatars (by inserting the url of image)

 

 

however i have noticed that people have been able to use urls from my website which will give them +rep

 

so what i wanna do is stop them being able to use the word mafiakillerz in the form

 

so something like

 

<?php

if($_POST['avatar'] includes "mafiakillerz")

{

echo "no";

}

?>

[mapleleaf]

if(strstr($_POST['avatar'],'mafiakillerz')) {
    echo 'No';
}