Run php function on button click

I tried the code of William, Thanks brother.

but it's not working as a simple button I have to add form with method="post". Also I have to write submit instead of button.

here is my code below..

<form method="post">
    <input type="submit" name="test" id="test" value="RUN" /><br/>
</form>

<?php

function testfun()
{
   echo "Your test function on button click is working";
}

if(array_key_exists('test',$_POST)){
   testfun();
}

?>

<a href="home.php?click=1" class="btn">Click me</a>
<?php 
  if($_GET['click']){
    doSomething();
  }
?>

But is better to use JS and with ajax to call function!

Do this:

<input type="button" name="test" id="test" value="RUN" /><br/>

<?php

function testfun()
{
   echo "Your test function on button click is working";
}
if(array_key_exists('test',$_POST)){
   testfun();
}
?>