How to execute Javascript function in PHP

I have a button which successfully triggers a "load more" on click. Once it is clicked, a SESSION variable is set, so that when the user reloads the page, the new posts should appear loaded already, so that the user does not need to click again "load more".

I would thus like to execute the same "load more" Javascript function in a PHP IF statement, according if a SESSION or COOKIE is set or not:

<?php
if ((isset($_SESSION['isloaded'])) || (isset($_COOKIE['isloaded']))){
echo '<script>loadmore(\'posts\');</script>';
}
else {
echo '<a href="#" onClick="return false" onmousedown="javascript:loadmore(\'posts\');" class="btn btn-primary" id="load-more-btn">Load More</a>';
}
?>

However the Javascript does not get triggered. What am I doing wrong?