odd that 5.3 is so different than 5.1.6
the two are rather close
first WHY do you think you NEED just that and ONLY that version ?
was their a very old and unused call that was removed from 5.1.7 and up ?
dose the documentation for this old program state to install ( the THEN current) 5.1.6
this IS red hat so you DO have the required support contract
have you asked red hat seeing as you ARE paying them $$$$ every year
if it is some function that was removed from newer versions of php
then install the old version from source
but that might " null and void" your Red hat license and contract
so ask them .