I want to get element value of current page in php

<input type="hidden" id="id" name="id" value="100">    
<? $id = '100'; ?>
<? $getid = echo '<script>document.getElementById("id");</script>'; ?>
<? if( $id == $getid ): ?>
<? echo 'id equal getid'; ?>
<? endif; ?>

I want to get a value using getElementById of current page in php.

but I can't get value.

document.getElementById("id");'; ?> code can't page load.

How can I fix it?