What does backslash do as prefix to a function name? [duplicate]

Question

Preparing for the ZEND-Exam, I found a question where a class redefined the strlen-function:

namespace MyFramework\MyString
{
  function strlen ($str) 
  {
    return \strlen($str) * 2; // return double string-length
  }

}

I never came across that "\function"-thing before and could not get an explanation from my trainer - can somebody pls. help shed some light...?

Answer 1

It calls a function from the global namespace.

You need it only if there's a function of the same name in the current namespace.