How to change this error message into alert pop up in php?
<p>I want to modify this code into pop up alert. Currently, this code only produce error message. I want to modify it so when course clashed, the pop up will appear</p>

<pre><code> if($clash_courses==1)
{
    $error_msg .= 'Course ';
    $c = 0;
    foreach($course_id as $id){
        $c++;
        $error_msg .= $id;
        if($c&lt;count($course_id))
        $error_msg .= ' and ';
    }

    $error_msg .= ' have clashed.';

}   
</code></pre>
