curry.php

<?PHP

class F {
    public static function curry($f) {
        return new F($f);
    }

    private function __construct($f, $args = []) {
        $this->f = $f;
        $this->args = $args;
    }

    public function _($arg = null) {
        if ($arg === null) {
            $f = $this->f;
            return $f($this->args);
        }

        $args2 = $this->args;
        $args2[] = $arg;
        return new F($this->f, $args2);
    }
}

$f = F::curry(function($_) {
        return $_[0] + $_[1];
    });

echo "result:\n";
echo $f->_(2)->_(3)->_() . "\n";
echo $f->_(3)->_(5)->_() . "\n";

$add2 = $f->_(2);
echo $add2->_(3)->_() . "\n";

<?PHP
echo $f->_(1)->_(2)->_(3)->_();

<?PHP
$fib = function($x) {
  return $x < 2
    ? $x
    : $fib($x-1) + $fib($x-2);
  })) // FAIL!!