If you are running PHP on Apache then you can use the enviroment variable called DOCUMENT_ROOT
. This means that the path is dynamic, and can be moved between servers without messing about with the code.
<?php
$fileLocation = getenv("DOCUMENT_ROOT") . "/myfile.txt";
$file = fopen($fileLocation,"w");
$content = "Your text here";
fwrite($file,$content);
fclose($file);
?>