In the source code of what? Can you dump the contents to a file and then search for the string within the file? Or if it's your file, you could probably just open the file as a stream and read it that way.
// The search string...
$search = "<param name=\"src\" value=\"http://n8.stagevu.com/v/0a7009bb38ae6bf96711298fd7161516/ckabnqdkxfla.avi\" />";
// The file location...
$file = "web_page.htm";
// Get a file handle...
$fh = fopen($file, "r");
// Read the file into a string...
$string = fread($fh);
if(strstr($string, $search))
// Do something...