Just a little snippet I created a while back and used recently, This function takes any number of bytes and returns it formatted as the "better looking" Unit.

Usage:

	string parse_size(number $size[,string $to][, bool $raw]);

Returns a string containing $size represented as $to followed by $to's suffix, if the optional value $raw is set to true returns a number.

Allowed values for $to are:

• b
• kb
• mb
• gb
• tb
• pb
  

Example:

parse_size(6789785445);
//6.32 GigaBytes 
 
parse_size(6789785445, 'mb');
//6,475.24 MegaBytes
 
parse_size(6789785445, 'mb', true);
//6,475.24

A working example can be found here

The code:

	//Parsea un numero dado en bytes a cualquier notaciÑn
	function parse_size($size, $to = false, $raw = false){
		$octal = true;
 
		$units = array(
			'b' => array(
				'name' => "Byte",
				'depth' => 0
			),
			'kb' => array(
				'name' => "KiloByte",
				'depth' => 1
			),
			'mb' => array(
				'name' => "MegaByte",
				'depth' => 2
			),
			'gb' => array(
				'name' => "GigaByte",
				'depth' => 3
			),
			'tb' => array(
				'name' => "TeraByte",
				'depth' => 4
			),
			'pb' => array(
				'name' => "PetaByte",
				'depth' => 5
			)
		);
		//Utilizamos escala octal (como en el sistema) o decimal (como en los discos duros)???
		$escala = ($octal)?1024:1000;
 
		if($to != false && !$units[$to]) return false;
		if(!$to){
			$loop = true;
			$to = 'kb';
		}
		if($size < $escala){
			$to = 'b';
			$parsed = $size;
			$s = ($parsed == 1)?"":"s";
		 	return number_format($parsed, 2)." {$units[$to]['name']}$s";
		 }
		$ss = pow($escala, $units[$to]['depth']);
		$parsed = round($size/$ss, 2);
		$s = ($parsed == 1)?"":"s";
		if($loop){
			$keys = array_keys($units);
			$i = 2;
			while($parsed > 1 && $i < 6){
				$previous = array(
					'value' => $parsed,
					'key' => $keys[$i-1]
				);
				$to = $keys[$i];
				$parsed = parse_size($size, $to, true);
				$i++;
			}
			if($parsed < 1){
				$parsed = $previous['value'];
				$to = $previous['key'];
			}
		}
		if($raw) return $parsed;
		return number_format($parsed, 2)." {$units[$to]['name']}$s";
	}

Just a friendly reminder. All the code I post in here is licensed under an MIT License unless explicitly stated.