PHP Versions are fully supported for 2 years. 
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After 2 years the version will then be updated for critical security risks only for 1 year
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It is then fully un-supported

^ PHP Version           ^ Supported Until ^ Critical Security Updates ^
| 5.3 Released 01.01.12 | 2014            |  NOT SUPPORTED         |
| 5.4 Released 01.03.12 | 14.09.14        |  Until 14.09.15        |
| 5.5 Released 20.06.13 | 10.06.15        |  Until 10.07.16        |
| 5.6 Released 28.08.14 | 28.08.16        |  Until 28.08.17        |

Source: http://php.net/supported-versions.php