The onclick event of html tag works for javascript but not for php code. But you can call a php function by onclick event within javascript code using Javascript document.write() function. In this way you can easily access a php function by onclick event of html tag within javascript code.Let see below
<?php function phpcluster(){ echo "Hello Vikash"; } ?> <input type="button" name="check" onclick="document.write('<?php phpcluster () ?>');" value="Check">