Speaking to its "relevancy", PHP is still by far the most widely used web language (see: WordPress).
There could be many reasons why KBin uses PHP, from general support across the widest range of platforms to accessibility of the language to facilitate extensibility, or even just because that's what they felt most comfortable developing in.
Generally speaking if code is behaving poorly, it's the code writer rather than the coding language itself.
Source: professional web developer with more than 15 years of experience