Add cache length parameter to the PHP parameter parsing.

The PHP plugin segfaults when passing a cache name parameter.

```
Mon Jul  7 12:14:36 2014 - !!! uWSGI process 3000 got Segmentation Fault !!!
Mon Jul  7 12:14:36 2014 - *** backtrace of 3000 ***
/usr/bin/uwsgi(uwsgi_backtrace+0x2e) [0x46028e]
/usr/bin/uwsgi(uwsgi_segfault+0x21) [0x460411]
/lib/x86_64-linux-gnu/libc.so.6(+0x35480) [0x7f6518d9c480]
/usr/lib/libphp5.so(+0x41786b) [0x7f65179c986b]
/usr/lib/libphp5.so(execute_ex+0x40) [0x7f65179983e0]
/usr/lib/libphp5.so(dtrace_execute_ex+0x68) [0x7f651794cad8]
/usr/lib/libphp5.so(zend_execute_scripts+0x170) [0x7f651795ed90]
/usr/lib/libphp5.so(php_execute_script+0x1eb) [0x7f65178fdfab]
/usr/lib/uwsgi/plugins/php_plugin.so(uwsgi_php_request+0x682) [0x7f6518128772]
/usr/bin/uwsgi(wsgi_req_recv+0xa4) [0x416d44]
/usr/bin/uwsgi(simple_loop_run+0xc4) [0x45c964]
/usr/bin/uwsgi(uwsgi_ignition+0x205) [0x460915]
/usr/bin/uwsgi(uwsgi_worker_run+0x2bd) [0x463b8d]
/usr/bin/uwsgi(uwsgi_run+0x3a2) [0x464092]
/usr/bin/uwsgi(_start+0) [0x41637e]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7f6518d88b45]
/usr/bin/uwsgi() [0x4163a7]
*** end of backtrace ***
```