memoize_fn.php

<?php

function memoize(Closure $fn) : Closure {

    return function (...$args) use ($fn) {
        static $results = [];
        $argsHash = sha1( join("", array_map('serialize', func_get_args())));
        $results[$argsHash] = $results[$argsHash] ?? $fn(...$args);

        return $results[$argsHash];
    };

}

// the original fn
$square = function (int $toSquare) { sleep(1); return $toSquare**2; };
// the memoized version
$memoizedSquare = memoize($square);

//echo $square(2).PHP_EOL;
//echo $square(4).PHP_EOL;

echo $memoizedSquare(2).PHP_EOL; // not set
echo $memoizedSquare(2).PHP_EOL; // set
echo $memoizedSquare(4).PHP_EOL; // not set
echo $memoizedSquare(2).PHP_EOL; // set
echo $memoizedSquare(4).PHP_EOL; // set
echo $memoizedSquare(5).PHP_EOL; // not set

$add = fn($a, $b) => $a + $b;
$memoizedAdd = memoize($add);

echo $memoizedAdd(3, 5).PHP_EOL;
echo $memoizedAdd(5, 5).PHP_EOL;
echo $memoizedAdd(5, 5).PHP_EOL;