Thanks for all the answers and stuff to read!
When I look at the wiki page Software Collections
http://wiki.contribs.org/Software_collections
this is exactly what I did. Install scl-utils, enable the CENTOS SCL repo and install php54.
Normally the way the SCL software package is invoked is with a scl enable command, which creates the right environment:
myserver #scl enable php54 'php -v'
PHP 5.4.16 (cli) (built: Jul 10 2014 10:06:38)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies
I think it should be possible to force apache to start using php54 environment. Using a cgi script has the benefit of enabling a single ibay (website) to use php5.4.