Thank you ~s.o.s~, you solved my problem in a pretty good manner.
I had kind of solved it this way:
by adding on the input tag the event handlers:
onMouseUp="setTimeout(function(){ winStop()},500);"
onKeyPress="setTimeout(function(){ winStop()},500);"
this way I called the JavaScript function winStop which is defined as follows:
function winStop() //function that stops the efforts of the web browser to show the results of each PHP file executed
{
if (navigator.appName=='Microsoft Internet Explorer') document.execCommand('Stop');
else window.stop();
} //Tested in both Firefox and Internet Explorer.
but
at first it was not such a nice solution and
at second there was a problem: this stop command whould stop not only the efforts of the web browser to show the results of the execution of the .php file but it also stopped the opening of the main web page that possibly had not finished loading, when the stop function was called.
So, thank you for solving in a much better way my problem.