If you are looking for an introduction to Support Vector Machines and PHP, I have a post about how to write a basic SVM solver in PHP, but for most people looking to just use SVM with PHP, take a look at the PHP SVM extension, available from PECL, and built on top of the excellent LibSVM library.
For convenience, and because a few people that have asked, you can download pre-made Windows 32 bit builds of the extension here as well. The zip includes threadsafe and non-threadsafe versions, and the libsvm.dll from LibSVM 3.11 - both have passed smoke tests, but please report any issues on the PHP-SVM Github repo.
- php-svm-0.1.9.zip - built 2012-11-23
Note that there are currently no 64bit builds available, but the build process is described in the README if you have a 64bit system and a little bit of time!