Generate a relative URL from path A to B

relative-path.php

<?php

function relativePath($from, $to) {
  $fromAry = explode('/', $from);
  $toAry = explode('/', $to);
  while (count($fromAry) && count($toAry) && $fromAry[0] === $toAry[0]) {
    array_shift($fromAry);
    array_shift($toAry);
  }
  $up = count($fromAry);
  if ($up > 0 && count($toAry)) $up--;
  $relative = str_repeat('../', $up) . implode('/', $toAry);
  if ($relative === '') $relative = basename($to);
  return $relative;
}

$tests = [
  ['a/b/c', 'a/b/c', 'c'],
  ['a/b/c', 'a/b/d', 'd'],
  ['a/b/c', 'a/b/d/g', 'd/g'],
  ['a/b/c', 'a/b', '../'],
  ['a/b/c', 'a', '../../'],
  ['a/b/c', 'e', '../../e'],
  ['a', 'a', 'a'],
  ['a', 'e', 'e'],
  ['a', 'e/f', 'e/f'],
];

foreach ($tests as $test) {
  list($from, $to, $expected) = $test;
  $relative = relativePath($from, $to);
  if ($relative !== $expected) {
    echo "Fail: $from -> $to => $relative != $expected\n";
  }
}