Why can't I declare a function name "_"?

Code:

function _()
{
    echo开发者_Go百科 'hello word';
}

Output:

Fatal error: Cannot redeclare _()

I haven't defined this function _ before, then why I am getting this error?

_() is an alias for gettext.

_ is an alias for gettext, a built-in function. Just as you cannot declare a function gettext, you can't create a function _ since one is already there.