const MAX_LENGTH = 256 * 1024;
$datasets = [
 'a' => ['%66%6F%6F', '', 'foo'],
 'b' => ['%66%6F%6F', 'o', 'f%6F%6F'],
 'c' => ['%66%6F%6F', 'f', '%66oo'],
 'd' => ['%66%6F%6F', 'fo', '%66%6F%6F'],
 'e' => ['%66%6f%6f', 'fo', '%66%6f%6f'],
 'no-escapes' => [str_repeat('a', MAX_LENGTH), '', str_repeat('a', MAX_LENGTH)],
 'escapes' => [str_repeat('%40', MAX_LENGTH / 3), '', str_repeat('@', MAX_LENGTH / 3)],
 'escapes2' => [str_repeat('%40', MAX_LENGTH / 6 - 3) . '%20' . str_repeat('%40', MAX_LENGTH / 6 - 3), '', str_repeat('@', MAX_LENGTH / 6 - 3) . '%20' . str_repeat('@', MAX_LENGTH / 6 - 3)],
 'escapes3' => [str_repeat('%40', MAX_LENGTH / 3), '@', str_repeat('%40', MAX_LENGTH / 3)],
 'escapes4' => [str_repeat('%40X', MAX_LENGTH / 4), '@', str_repeat('%40X', MAX_LENGTH / 4)],
 'escapes5' => [str_repeat('%40X%41Y', MAX_LENGTH / 8), '@', str_repeat("%40XAY", MAX_LENGTH / 8)],
];
$tests = [
 'old' => function ($testCount, $s, $reserved, $expected) {
 for ($i = 0; $i < $testCount; $i++) {
 $actual = $s;
 preg_match_all('#(?<=%)[a-f0-9][a-f0-9]#i', $s, $matches, PREG_OFFSET_CAPTURE | PREG_SET_ORDER);
 foreach (array_reverse($matches) as $match) {
 $ch = chr(hexdec($match[0][0]));
 if (strpos($reserved, $ch) === FALSE) {
 $actual = substr_replace($actual, $ch, $match[0][1] - 1, 3);
 }
 }
 }
 Assert::same($expected, $actual);
 },
 'new' => function ($testCount, $s, $reserved, $expected) {
 for ($i = 0; $i < $testCount; $i++) {
 $actual = $s;
 preg_match_all('#%[a-f0-9][a-f0-9]#i', $s, $matches, PREG_OFFSET_CAPTURE | PREG_SET_ORDER);
 foreach (array_reverse($matches) as $match) {
 $ch = chr(hexdec(substr($match[0][0], 1)));
 if (strpos($reserved, $ch) === FALSE) {
 $actual = substr_replace($actual, $ch, $match[0][1], 3);
 }
 }
 }
 Assert::same($expected, $actual);
 },
 'split' => function ($testCount, $s, $reserved, $expected) {
 for ($i = 0; $i < $testCount; $i++) {
 if ($reserved === '') {
 $actual = rawurldecode($s);
 } else {
 $pattern = '#((?:%(?:' . implode('|', str_split(bin2hex($reserved), 2)) . '))++)#i';
 $parts = preg_split($pattern, $s, -1, PREG_SPLIT_DELIM_CAPTURE);
 $max = count($parts);
 for ($j = 0; $j < $max; $j += 2) {
 $parts[$j] = rawurldecode($parts[$j]);
 }
 $actual = implode('', $parts);
 }
 }
 Assert::same($expected, $actual);
 },
 'split-limited' => function ($testCount, $s, $reserved, $expected) {
 for ($i = 0; $i < $testCount; $i++) {
 if ($reserved === '') {
 $actual = rawurldecode($s);
 } else {
 $actual = $s;
 $pattern = '#((?:%(?:' . implode('|', str_split(bin2hex($reserved), 2)) . '))++)#i';
 $res = array();
 do {
 $parts = preg_split($pattern, $actual, 1024, PREG_SPLIT_DELIM_CAPTURE);
 $actual = isset($parts[2046]) ? $parts[2046] : '';
 unset($parts[2046]);
 $max = count($parts);
 for ($j = 0; $j < $max; $j += 2) {
 $parts[$j] = rawurldecode($parts[$j]);
 }
 $res[] = implode('', $parts);
 } while ($actual !== '');
 $actual = implode('', $res);
 }
 }
 Assert::same($expected, $actual);
 },
 'replace_callback' => function ($testCount, $s, $reserved, $expected) {
 for ($i = 0; $i < $testCount; $i++) {
 if ($reserved === '') {
 $actual = rawurldecode($s);
 } else {
 $pattern = '#(?:%(?!' . implode('|', str_split(bin2hex($reserved), 2)) . ')[0-9a-f]{2})++#i';
 $actual = preg_replace_callback(
 $pattern,
 function ($m) { return rawurldecode($m[0]); },
 $s
 );
 }
 }
 Assert::same($expected, $actual);
 },
];
$testCount = 10;
$padLength = max(array_map('strlen', array_keys($datasets))) + 3;
set_time_limit(0);
ini_set('memory_limit', '512M');
foreach ($tests as $testName => $testCallback) {
 printf("Test %s:\n", $testName);
 foreach ($datasets as $datasetName => $dataset) {
 $time = -microtime(TRUE);
 $testCallback($testCount, ...$dataset);
 $time += microtime(TRUE);
 printf(" %s%5.0f ms\n", str_pad($datasetName, $padLength), $time * 1e3);
 }
 printf("\n");
}

Test old:
 a 0 ms
 b 0 ms
 c 0 ms
 d 0 ms
 e 0 ms
 no-escapes 84 ms
 escapes 11424 ms
 escapes2 12724 ms
 escapes3 2238 ms
 escapes4 1685 ms
 escapes5 8341 ms
Test new:
 a 0 ms
 b 0 ms
 c 0 ms
 d 0 ms
 e 0 ms
 no-escapes 1 ms
 escapes 14472 ms
 escapes2 15174 ms
 escapes3 2349 ms
 escapes4 1748 ms
 escapes5 13632 ms
Test split:
 a 0 ms
 b 0 ms
 c 1 ms
 d 0 ms
 e 0 ms
 no-escapes 4 ms
 escapes 71 ms
 escapes2 118 ms
 escapes3 29 ms
 escapes4 670 ms
 escapes5 375 ms
Test split-limited:
 a 0 ms
 b 0 ms
 c 0 ms
 d 0 ms
 e 0 ms
 no-escapes 3 ms
 escapes 71 ms
 escapes2 117 ms
 escapes3 30 ms
 escapes4 524 ms
 escapes5 314 ms
Test replace_callback:
 a 0 ms
 b 0 ms
 c 0 ms
 d 1 ms
 e 0 ms
 no-escapes 3 ms
 escapes 72 ms
 escapes2 107 ms
 escapes3 50 ms
 escapes4 36 ms
 escapes5 309 ms