Built In Types: Resources

A resource is a descriptor to some sort of external entity. (Examples include files, databases, and sockets.) Resources are only created or consumed by the implementation; they are never created or consumed by Hack code. Each distinct resource has a unique ID of some unspecified form.

When scripts execute in a mode having a command-line interface, the following predefined resource-like constants that correspond to file streams are automatically opened at program start-up:

• STDIN, which maps to standard input
• STDOUT, which maps to standard output
• STDERR, which maps to standard error

These constants have some unspecified type, which behaves like a subtype of type resource.

Resources are a carryover from PHP, and their use is discouraged in Hack code.