I did this for a chat program once. You dont need a toolkit. Just parse the text thats entered (ie, separate by spaces) and keep the words in an array. Then start working through them, looking at the words and any subsequent words. Match the sequences with your dialect/slang, and for any match, do the substitution. So as you work through the words, you'd build the output string either from the original word, or from any substitutions you found.
Might help to maintain two arrays - one that maintains the case and punctuation, and the other that converts text to lower case to make your checks against the dictionary simpler. Make sure their array indexes match. This way when you dont find a match in the array with lower case text, you can output the original, unmodified text.
For the dictionary, in PHP, you can take advantage of its arrays - perhaps build it like
$words['find'] = 'replace'. Or for multi-word sequences, make it a nested array.
That should get you a decent first pass at a translator. You'll probaby then have to consider how to handle things like "text in quotes" and other punctuation.