<input type="button" onclick="jMail()" value="Send Mail"/>        
<script>
function jMail()
{
alert("Inside Javascript Function");
alert("<?php PHPFunction(); ?>");
}
</script>

<?php
function PHPFunction(){
echo ("Inside PHP Function");
mail("to@example.com","Help Mee","Definitely Help me","From:from@example.com");
echo ("Mail Sent");
}
?>