where to add the php file that related to the HTML page

Hi

i have html file and php file related to each other (must be together in same folder to work).

i made new page in wordpress and past the html code in it,
now there is related php file it must be in the same place that i put the html file so it will work…

where to put the PHP file?

this is part of the html code:

}
xmlhttp.open(“GET”,”getuser.php?q=”+str,true);
xmlhttp.send();
}