COBOL itself isn't tied to any specialized hardware, and you can get COBOL compilers (whether proprietary or open source) for all major platforms. If it runs PHP, it probably can run COBOL too.
The real problem with porting most COBOL code is that few COBOL applications are actually pure COBOL, they are full of calls to proprietary middleware, databases and operating systems. For example, a lot of COBOL code runs on IBM mainframes and calls mainframe packages like CICS and IMS and the underlying mainframe OS (z/OS aka MVS). While there are replatforming solutions which emulate some of these mainframe software packages on other platforms, the emulation is generally imperfect hence there is still effort and risk in migrating.
I think the biggest problem with mainframe COBOL software is not the hardware but finding people with the right skills to maintain it. Even if you use some replatforming solution to port it to a more mainstream platform (Linux/Unix/Windows), you will still need COBOL programmers familiar with the relevant mainframe technologies, plus they'll need to understand the limitations of the replatforming solution you chose.