PHP 5.5 is already in the “security support only” category:
Each release branch of PHP is fully supported for two years from its initial stable release. During this period, bugs and security issues that have been reported are fixed and are released in regular point releases.
After this two year period of active support, each branch is then supported for an additional year for critical security issues only. Releases during this period are made on an as-needed basis: there may be multiple point releases, or none, depending on the number of reports.
From PHP: Supported Versions.
The issue of how long to support previous releases is a big deal. There is already plenty of work to do in terms of pushing forward on new releases, working on previous releases just adds to the stack.
When possible the biggest help to everyone is making it clear when and for how long previous releases are going to be supported. This is something that Ubuntu has done a good job at. They established a pattern of LTS ( Long Term Support ) vs regular releases, making the expectations clear ( though their move to reduce non-LTS release support from 18 to 9 months definitely caused confusion/problems for people ).
If you want your favorite project to support previous releases longer, understand that means getting involved and providing resources ( time, money, people ).