The aspell() functions allows you to check the spelling on a word and offer suggestions.
aspell works only with very old (up to .27.* or so) versions of aspell library. Neither this module, nor those versions of aspell library are supported any longer. If you want to use spell-checking capabilities in PHP, use pspell instead. It uses pspell library and works with newer versions of aspell.
You need the aspell library, available from: http://aspell.sourceforge.net/.
See also pspell.