Q:

Replace text across website?

I help manage a nonprofit website with a large number of html files, probably around 1,000. Broken links are a hassle, and I was wondering if it would be possible, perhaps with PHP, to create a program that replaces a text string (ie a URL) across all of these files. 
Would it create a lot of server strain with this many files?
What about if, say 10 people, were simultaneously using the program to find and replace across all 1,000 files?

A:

It certainly is possible!
The key to this is to be able to read a file in PHP, and to process its contents. Reading is dead-easy, so is writing. Editing is the bit that'll require your attention.
$yourFilename = "file.html";
$yourFileData = file_get_contents($yourFilename);
if ($yourFileData !== false) {
     // File has been read successfully
     // $yourFileData has the contents of it. This is where the magic needs to happen!
}
// Write everything back
$file = fopen($yourFilename,"w");
fwrite($file,$yourFileData);
fclose($file);

This snippet of code effectively reads a file, and writes a file. The bit in the middle is where you'll do your search-and-replace. I strongly recommend using regular expressions for this. preg_replace will come in very handy for this, I am sure. Read up on it, and feel free to edit your question with more details - I'm more than happy to write the regular expressions for you.
Iterating through a directory is also pretty straightforward. You want to open a directory using opendir() and loop through using readdir(), making sure that the return value is not ., .. or false, and is a file.
For your additional question re:multiple users, I'd avoid if I were you. This creates race conditions.

