How can I convert a PHP date time string to milliseconds?

Question

<?php 
        date_default_timezone_set("Europe/London");
        $info = getdate();
        $date = $info['mday'];
        $month = $info['mon'];
        $year = $info['year'];
        $hour = $info['hours'];
        $min = $info['minutes'];
        $sec = $info['seconds'];
    ?>

Using the above I plan to concatenate this into a string of something like this:

"09/23/2000 9:45:00"

What I need in PHP is to convert the above string into milliseconds to pass to a JavaScript function.