How do I write a selective execution statement?

My a.php page is the Edit content page, which is an ordinary edit, then submits the content to the database page, the approximate structure is as follows
。。。
$sql = "Update ' usercontents ' Set ....
。。。


I want to implement this function, when the user click the Modify button, then execute the following statement, such as no point to modify, then do not.

$myfile = "1.txt";
Unlink ($myfile);
echo "";
?>

After I insert the statement into the UPDATE statement, I can't seem to execute it. How do you write it? Thank you, great God!!!

Reply to discussion (solution)

Is anyone there? Did I say something a little bit around?

As soon as you click Edit, the a.php will be executed (not executed without clicking)
I don't know what you want.

Thank you xuzuning reply. A.php is the name of the edit page.
The front of the page is the normal edit, submit the form and so on. What I want to do is, after clicking on the changes, I can execute the code above.

I tried to put the code after the update, but it didn't seem to work, and when I put this piece of code at the bottom of the page, the code was executed as soon as the a.php was opened, but it was not achieved: the effect was clicked and then executed.

