Guides

<?php
function add_as_string($a,$b){
	// Produce an error if either $a or $b is not a string.
	if(!is_string($a) || !is_string($b)){
		return "At least one of the provided numbers was not a string!";
	}
 
	// Finds the length of $a or $b, determined by which of $a and $b is the longest string.
	$length = strlen($a)>strlen($b)?strlen($a):strlen($b);
 
	// Zero-pads the strings if neccesary.
	$a = sprintf("%0{$length}s", $a);
	$b = sprintf("%0{$length}s", $b);
 
	// Here the actual addition takes place. Note that it does the addition "backwards".
	for($x=$length-1;$x>=0;$x--){
		$c[$x] += $a[$x]+$b[$x];
		if($c[$x] >= 10){
			$c[$x-1] += 1;
			$c[$x] -= 10;
		}
	}
	$c = array_reverse($c); // Reverse the array containing the addition numbers.
 
	// Puts all the numbers in $c into a string.
	foreach($c as $number){
		$result .= strval($number);
	}
 
	return $result;
}
 
$a = pow(2,1023); // the highest value php can give me. (corresponds to the maximum value of a 1024-bit signed integer)
$a = sprintf("%1.0f",$a); // makes $a a string.
 
// Here we let the parser give the addition a try. Note that this gives INF (meaning infinity).
echo $a+$a . "<br/>";
 
// We try to add the number via the add_as_string function.
// Note that when $a==$b, every loop of the for-loop is equal to multiplication with two. Meaning that if you run the loop twice, it's the same as 4*$a, and so fourth.
for($x=0;$x<1;$x++){
	$a = add_as_string($a,$a);
}
// We output the value of the 
echo $a . "<br/>";
 
// We try putting through two number's that are unequal in length, therefore making use of the zero-padding.
$a = add_as_string("5","50");
echo $a . "<br/>";
 
// We try putting through a number and a string, which will provide an error.
$a = add_as_string(5,$a);
echo $a . "<br/>";
?>