Parameters in PHP documentation, squared brackets?

Question

This function for example...

int preg_match_all ( string $pattern , string $subject [, array &$matches [, int $flags = PREG_PATTERN_ORDER [, int $offset = 0 ]]] )

It takes the strings $pattern and $subject. However, what is [?

Answer 1

That is how the documentation for a function indicates arguments that are optional