Question

What is an efficient way to get the part of a string after the occurrence of a certain needle, only if that needle is at the start of the haystack. Similar to strstr(), but excluding the needle, and only when found at the beginning of the string.

If it isn’t found, it should preferably return false.

I have the feeling I’m overlooking some very obvious PHP functions here, but I can’t seem to think of them right now.

For example:

$basePath = '/some/dir/';

$result = some_function( '/some/dir/this/is/the/relative/part', $basePath );
/*
should return:
this/is/the/relative/part
*/

$result = some_function( '/fake/dir/this/is/the/relative/part', $basePath );
$result = some_function( '/pre/some/dir/this/is/the/relative/part', $basePath );
/*
last example has '/some/dir/' in it, but not at start.
should both preferably return:
false
*/

I’ll be using this for a filesystem service that should act as a sandbox, and should be able to give out and take in paths, relative to the base sand box directory.

Answer 1

Put more simply than the other examples:

function some_function($path,$base){
  $baselen = strlen($base);
  if (strpos($path,$base) === 0 && strlen($path)>$baselen)
    return substr($path,$baselen);
  return false;
}

DEMO

Alternate using strncmp, too: DEMO