PHP脚本从顶部重新启动

I have a long-running php script that does some ffmpeg stuff. It works perfectly fine on my dev box (amazon ec2). But on production, (clone of the dev box), the php script resets from the top after 60 seconds.

<?php
function convert() {
    error_log('Executing ffmpeg');
    shell_exec(ffmpeg.....);
}
?>

so calling this service while looking at the error log and top. i get

20:32:04 Executing ffmpeg
20:33:04 Executing ffmpeg
20:34:04 Executing ffmpeg

and in top i get 3 ffmpeg

Any idea what in php would cause it to reset from the top?