Thank you jabdulla!
I looked at the code just above, then at the PHP and MySQL documentation, and learned something about soundex (PHP and MySQL, but different), the importance of adding a letter in front (e.g. Q), levenshtein, metaphone and other strange animals.
As my keywords are arranged in a tree, thesaurus-like, I might suggest the user to search with a broader term (e.g. "flatfish", in case a search on "sole" did not return any results) rather than a sound-alike ("shark" instead of "carp"?). Also, the user clicks in the keyword tree, which means that misspellings (MESSING PILLS? SMELL PIG SINS? MINGLES SLIPS?) do not occur all that often (no need for ELEVENTH SIN). harsh reality...
http://www.wordsmith.org/anagram/index.html