Example Error Message
PHP Fatal error: Call to undefined function posix_getpwuid() in /path/to/your/file.php on line 5
PHP posix extension is not available. Check this by invoking this command in your shell :
$ php -m | grep -i posix
If this output
posix then you have posix extension installed.
How to fix
Should be enabled by default when you install php from apt-get.
Need php-process extension installation (the package name is depend on your system, usually started with php-process).
$ sudo yum install php-process.x86_64
Based on PHP documentation. This compile option is enabled by default.