Sorry, but the function str_reverse is not in the online manual. Perhaps you misspelled it, or it is a relatively new function that hasn't made it into the online documentation yet. The following are the 20 functions which seem to be closest in spelling to str_reverse (really good matches are in bold). Perhaps you were looking for one of these: array_reverse ccvs_reverse strrev <=== ja hoor daar hebben we hem al :-)
Uit het oogpunt van performance kun je misschien beter eerst in PHP de string omkeren dan dat je het door MySQL laat doen, maar dat zou je even moeten uittesten.