The Y (and U) combinator(s), with an example, implemented in PHP.

example.php

<?php

$fib = function ($f) {
  return function ($n) use ($f) {
    switch ($n) {
      case 0: return 0;
      case 1: return 1;
      
      default:
        return $f ($n - 1)
             + $f ($n - 2);
    }
  };
};

Y ($fib) (10);

Y.php

<?php

function U ($f) {
  return $f ($f);
}

function Y ($f) {
  return U (function ($x) use ($f) {
    return $f (function ($y) use ($x) {
      return U ($x) ($y);
    });
  });
}