Why wouldn't this work? This is just an example but how would I echo PHP using onClick()?
<script type="text/javascript">
var check = <?php echo 'lol'; ?>;
</script>
<input type="submit" value="click me" onClick="document.write(check);">
Look at the output of your PHP:
var check = lol;
lol is an undefined variable.
If you want to generate JavaScript with a string in it, then you have to generate the syntax for a string literal.
The json_encode function will do this (while escaping any characters that need escaping for JS).
var check = <?php echo json_encode('lol'); ?>;
If you are using a .php file
<script type="text/javascript">
var check = '<?php echo "lol"; ?>';
</script>
<input type="submit" value="click me" onClick="document.write(check);">
If you are not using a .php file then php code will not work.
<script type="text/javascript">
var check = 'lol';
</script>
<input type="submit" value="click me" onClick="document.write(check);">