Ahead of referencing it at my PHP Tek 11 talk, I finally got round to pushing the LibSVM extension I knocked up (with the help of Mikko) into PECL. You'll need libsvm out of package management (e.g. apt-get install libsvm-dev or yum install libsvm-devel), and the pecl package to try it:
pecl install svm-beta
The documentation is in the PHP manual, and being that it's fairly early days I would really appreciate anyone that has familiarity with SVM to have a go with the extension, and let me know how it goes, either on twitter, via email (ian dot barber at gmail dot com will work), or of course in the PECL bugtrackers.