http://www.php.net/manual/en/ref.pspell.php
aspell, an open source library, includes pspell support for php. You can get it at
aspell.sourceforge.net or
www.aspell.net. I believe, although I may be incorrect, that ispell was originally intended for use with perl. Installing the aspell libraries is all you need to do. The aspell developers call it "A More Intelligent Ispell".
Installing it depends entirely on your server platform, version of php, and the level of access you have with your server. If you host your own site on your own server that you have full, unrestricted access to, then you should understand how to add libraries to php. If you have no idea how any of this works, chances are you have a shared server or managed account that you're paying someone else to maintain for you. If that's the case, contact your hosting company's support staff and request that pspell support be compiled into php by compiling with the --with-pspell[=dir]. Direct them to the aspell.sourceforge.net site as well.
If they do not understand how to do this, or are unwilling to do this for you, then take this as a wake up call that you need a more competent host and switch to a company that is more responsive to your needs and is more familiar with the technology they are using to host your web site. It might seem like more trouble than it's worth for a spell checker, but in all honesty, if they don't understand this stuff, they're in the wrong business.
You cannot add pspell support to php without root access to your machine. If you are mailing out a monthly check to some company for their "web hosting plan" then chances are you cannot do this yourself, unless you have a dedicated server with root access.