In a previous article, Timothy Boronczyk wrote about how to create watermarks with Imagick using an overlay image. In this article, I'll show you how to achieve a similar effect using plain text.

He takes a sample image (a headshot) and offers two versions of the same functionality - both the shell command (using "convert") and the PHP code that sets up the font to use, adds placement and pushes the result back out as a PNG. He also includes two other types of overlay - a font mask for a more see-through look and a tiled version, overlaying the text all over the image.

PHP internally uses hashtables to store arrays. The above creates a hashtable with 100% collisions (i.e. all keys will have the same hash). [...] Because every hash function has collisions this C array doesn't actually store the value we want, but a linked list of possible values. [...] Normally there will be only a small number of collisions, so in most cases the linked list will only have one value. But the [included script] creates a hash where all elements collide.

He explains why it works, noting that it's relatively simple to do in PHP because of how it applies a table mask. The slowness comes in when PHP is forced to go through the entire list when it tries to insert. Because of this issue, there's the potential for a Denial of Service attack that could potentially take a server down. There's a fix already in place for the problem, though, so keep an eye out for the next release (that will include a max_input_vars setting to prevent it).