How do I change the assignment by clicking on a cell?

Dear God, I would like to achieve click on a cell, the contents of the cell and the assignment of the other variable is changed, I wrote, it seems not possible, why? Thank you!
$txt = ' no '; $m = 17;
echo "

". $txt."

";

function ad () {
if ($txt = = ' no ')
{$txt = ' have ';
$m = 21;}
Else
{$txt = ' no ';
$m = 17;}
}

?>
In fact, $txt is two values, one is "no", one is "have", $m is two values, one is 17, and the other is 21.

Reply to discussion (solution)

Of course not!

This is called the JS function ad, not the PHP function ad

How do you change that? I still can't! Thank you, great God.

Do you have a look for me? How to achieve this ah.

You write the function ad () in the script tag followed by the Echo statement. The Click Call is not a PHP function.

It's still not working. I actually want to implement the Click cell, then the content is switched between 2 values, somehow.

