Michael Maclean has a look at working with fonts in Cairo (a graphics manipulation extension in the PECL repository). The example will be added to the PHP manual too in case you need it for reference later.
Currently, in PECL/Cairo the only way to draw text is the referred to as the "toy" text API, which is a very basic way of handling text compared to the facilities available in the Cairo library itself. However, it's sufficient for most purposes that I've come across so far.
With the first versions of the Cairo extension, you had to use the selectFontFace method to choose the font you wanted. With the introduction of Cairo 0.2.0, you can now use FreeType font support instead and can use the streams API built into PHP to locate the resource. A snippet of code is included to show how that would work.