Description:
------------
---
From manual page: http://www.php.net/function.sort
---
I don't really think that it's problem in the sort, but you need to fix documentation with adding information about sorting strings as numeric when values contains numbers only.

Test script:
---------------
$a = ["1500", "151"];
foreach($a as $item) { echo "Value is " . gettype($item) . PHP_EOL; }
echo "default:" . PHP_EOL;
sort($a);
var_dump($a);
echo "regular:" . PHP_EOL;
sort($a, SORT_REGULAR);
var_dump($a);
echo "numeric:" . PHP_EOL;
sort($a, SORT_NUMERIC);
var_dump($a);
echo "string:" . PHP_EOL;
sort($a, SORT_STRING);
var_dump($a);

Expected result:
----------------
Value is string
Value is string
default:
array(2) {
  [0] =>
  string(4) "1500"
  [1] =>
  string(3) "151"
}
regular:
array(2) {
  [0] =>
  string(4) "1500"
  [1] =>
  string(3) "151"
}
numeric:
array(2) {
  [0] =>
  string(3) "151"
  [1] =>
  string(4) "1500"
}
string:
array(2) {
  [0] =>
  string(4) "1500"
  [1] =>
  string(3) "151"
}


Actual result:
--------------
Value is string
Value is string
default:
array(2) {
  [0] =>
  string(3) "151"
  [1] =>
  string(4) "1500"
}
regular:
array(2) {
  [0] =>
  string(3) "151"
  [1] =>
  string(4) "1500"
}
numeric:
array(2) {
  [0] =>
  string(3) "151"
  [1] =>
  string(4) "1500"
}
string:
array(2) {
  [0] =>
  string(4) "1500"
  [1] =>
  string(3) "151"
}