parse-size.php

<?php

$value = '4 mb';

$units = ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
$aliases = ['k' => 1, 'kiB' => 1, 'm' => 2, 'MiB' => 2, 'g' => 3, 'GiB' => 3, 't' => 4, 'TiB' => 4, 'PiB' => 5, 'EiB' => 6, 'ZiB' => 7, 'YiB' => 8];

preg_match('/([0-9,.]+)\s*([a-z]+)/i', $value, $matches);

[$_, $size, $unit] = $matches;

$size = (float) str_replace(',', '', $size);

$lookup = collect($units)
	->combine(array_keys($units))
	->union($aliases);

$exponent = $lookup->first(fn($exponent, $key) => 0 === strcasecmp($key, $unit));
$multiplier = 1000 ** $exponent;
$bytes = $size * $multiplier;

$unit = $units[$exponent];

dump("'{$value}' was normalized to '{$size}{$unit}' and converted to '{$bytes} bytes'");