String with function

string-eval.php

<?php

function day($value) {
    return date('d', strtotime($value));
}

function equal($one, $second) {
    return $one == $second;
}

$singleData = [
    "create_at" => "'2019-10-5'",
    "updated_at" => "'2019-10-10'"
];

$string = "equal(day(create_at) * 2, day(updated_at))";

$result = eval('return '.str_replace(array_keys($singleData), array_values($singleData), $string).';');

var_dump($result); // return true;