PHP Variables lose when "Edit JavaScript fragment" Follow

Hello,
I have a PHP script where a Javascript heredoc example:

Registry::getInstance()->jsOnReady .= <<<JS

var fee_count = $cntprcfee;

......

when I do a "Edit JavaScript fragment" and go back to my php script I lose the php variables incrusted in JavaScript code like this :

Registry::getInstance()->jsOnReady .= <<<JS

var fee_count = _cntprcfee;

how can I prevent this from happening.

Thanks in advance.