This code sample tries to find perfect numbers using the „Brothers in Binary“ theorem. For more information visit: http://www.futilitycloset.com/2013/04/09/brothers-in-binary/

brothers_in_binary.php

<?php
// Loop until the 20th binary digit of 0
for ($bin = 1; $bin < 20; $bin++)
{
	// 1 has always one digit more than 0
	$bin_str = str_repeat('1', $bin +1) . str_repeat('0', $bin);
	
	// Convert binary to decimal
	$dec = bindec($bin_str);
	
	// Create all dividers
	for ($div = 1; $div < $dec; $div++)
	{
		// Save only integer-dividers
		if ($dec % $div == 0) { $int_arr[] = $div; }
	}
	
	// Check if integers are available
	if ($int_arr)
	{
		// Check if perfect number by summing up all integers and matching against the decimal
		if (array_sum($int_arr) === $dec) { echo($dec . ' - ' . $bin_str . "\n"); }
		
		// Delete array before next iteration
		unset($int_arr);
		
	}
	
}