singletonize_example_use.php

<?php
class ClassThatMarksDateOfInstantiation
{
    private $dob;
    public function __construct()
    {
        $this->dob = new \DateTimeImmutable;
    }
    public function getDob()
    {
        return $this->dob;
    }
}

$factoryOfDobClass = function () {
    return new ClassThatMarksDateOfInstantiation;
};

$DobSingleton = singletonize($factoryOfDobClass); // -> anonymous class
$dob = new $DobSingleton; // -> new instance of anonymous class
$dob2 = new $DobSingleton; // -> same instance of anonymous class
print_r($dob->getDob() === $dob2->getDob()); // -> 1