Save searched words in a txt file

Hi, i have a normal search function in my site, and i want to save any searched words typed in the search box in a .txt file (like: search.txt).
each word in a seperate line like this sample.txt

--------------------------------------------
new cars
how make money
new cars
etc.
--------------------------------------------

This is my search form:

<form  onsubmit="search(src.value);return false;" id="searchform" method="get">
                <input type="text"  value=""  id="s" name="src"  />
                <a onclick="document.getElementById('searchform').submit(); return false;" id="submit-search" href="#"><span>search</span></a>

                <input type="submit" name="submit" style="display: none;" value="search word"/>
            </form>

Code (markup):

by which php code can i save the searched words in this txt file?

 

<form method="get">
<input type="text" name="src">
<input type="submit" name="submit" value="Search">
</form>

<?php

if(isset($_REQUEST['submit']) && !empty($_REQUEST['src'])){
$fp = fopen('searched.txt',"a");
fwrite($fp,$_REQUEST['src']."\n");
}

?>

PHP:

 

thx
where should i paste this code? the search form is in a .tpl file. and it does not work when pasting in there
should i create a seperate .php file? if yes how should i point it to the .tpl file?

 

are you have search.tpl or similar to parse that search text?