[CentOS] php-5 install issues on centos 4

OK, i've found php-5 RPMs in the centosplus dir on mirrors.

Sorry for my ignorance, but what's the difference between

      php-5.1.6-1.2.1.centos.x86_64.rpm
and
      php-5.1.6-3.el4s1.5.x86_64.rpm

?
There's other such examples in the same dir.

Anyway, i opted to try installing the "el4" variety, along with the 
comparable RPM for php-mysql, but --after satisfying other dependencies-- 
ran into this:

    libmysqlclient.so.15()(64bit) is needed by php-mysql-5.1.6-3.el4s1.5.x86_64
    libmysqlclient.so.15(libmysqlclient_15)(64bit) is needed by php-mysql-5.1.6-3.el4s1.5.x86_64

Which stumps me, because the libmysqlclient14-devel RPM would seem to be 
the latest-greatest available in that centosplus dir.  Does one have to 
turn elsewhere for the libmysqlclient15-devel RPM that would presumably 
supply the right SO file?


Thanks for any & all help.


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