How to call php file in theme directory from plugin

I resolved my issue.
My error came from both calling the php file from the wordpress theme and converting the html code to an object value so that jquery ajax interprets it as it should.

$tmpdir = get_template_directory() . '/ajax_user.php';
$resumeaccount = file_get_contents($tmpdir);

So I get Html content inside singles quote (value object) instead of directly interpreting the html code