Regular expressions (which will now be referred to as "regexes") are basically pattern matching inside of text. They use special syntax and concepts in order to obtain information from a string. Many programming languages have some sort of support for regexes, because of the sheer usefulness of them. Not only can patterns be used to validate that a certain pattern exists in a string, but they can also be used to physically extract matched portions and make them usable in your PHP code.

They look at some of the basics - example structure, metacharacters, greediness as well as include a brief comparison of PCRE versus POSIX versions. The tutorial is peppered with plenty of example expressions too, giving you a good idea of what you're working towards knowing.

is_callable() will return TRUE even if a function has been disabled in php.ini with disabled_functions (which, in my mind, is contrary to what '�is_callable'� implies). function_exists() will return FALSE if the function is disabled in php.ini, but will return TRUE if the function is being blocked by safe_mode.

So, to help counter these issues, he checks first to see if the exec('id') option works - checking it work function_exists and the setting for safe_mode (disabled). If it doesn't pass, he moves on to the posix_* function and tests then with a function_exists.