Javascript-Like Time Function

[code?]

How do I get a javascript-like output in seconds, from PHP? I want to track the amount of time it takes to pass all the data from server to client.

Consider this code.

var Now = new Date().getTime();
document.write(new Date().getTime() - Now);

I want it like this.

<script>
    <?php
        ????
        echo "var _requestTime = " .  ???? . ";";
    ?>
    var RequestTime = new Date().getTime() - _requestTime;
</script>

Help me fill in the blanks.