Find the longest common substring in an array of strings (PHP)

longest_common_substring.php

<?php
function longest_common_substring($words)
{
  $words = array_map('strtolower', array_map('trim', $words));
  $sort_by_strlen = create_function('$a, $b', 'if (strlen($a) == strlen($b)) { return strcmp($a, $b); } return (strlen($a) < strlen($b)) ? -1 : 1;');
  usort($words, $sort_by_strlen);
  // We have to assume that each string has something in common with the first
  // string (post sort), we just need to figure out what the longest common
  // string is. If any string DOES NOT have something in common with the first
  // string, return false.
  $longest_common_substring = array();
  $shortest_string = str_split(array_shift($words));
  while (sizeof($shortest_string)) {
    array_unshift($longest_common_substring, '');
    foreach ($shortest_string as $ci => $char) {
      foreach ($words as $wi => $word) {
        if (!strstr($word, $longest_common_substring[0] . $char)) {
          // No match
          break 2;
        } // if
      } // foreach
      // we found the current char in each word, so add it to the first longest_common_substring element,
      // then start checking again using the next char as well
      $longest_common_substring[0].= $char;
    } // foreach
    // We've finished looping through the entire shortest_string.
    // Remove the first char and start all over. Do this until there are no more
    // chars to search on.
    array_shift($shortest_string);
  }
  // If we made it here then we've run through everything
  usort($longest_common_substring, $sort_by_strlen);
  return array_pop($longest_common_substring);
}
?>

longest_common_substring_example.php

<?php
$array = array(
  'PTT757LP4',
  'PTT757A',
  'PCT757B',
  'PCT757LP4EV'
);
echo longest_common_substring($array);
// => T757
?>

@sasivarnakumar glad you found it useful after all this time!

There is a bug

$a='Islas Vírgenes Británicas';
$b='Islas Vírgenes de EE. UU.';

$test = array($a, $b);

$foo = longest_common_substring($test);

print($foo);
// prints islas vírgenes opposed to Islas Vírgenes

Any suggestions?

@AliceWonderMiscreations Looks like an issue with UTF-8 characters. PHP has a set of multibyte functions that you could use in place of their non-multibyte versions in the function. See http://php.net/manual/en/ref.mbstring.php

Great function! Thank you!

to make it work with PHP8.x .. replace the create function line with:

16 $sort_by_strlen = function($a, $b) {
17 if (strlen($a) == strlen($b)) { return strcmp($a, $b); } return (strlen($a) < strlen($b)) ? -1 : 1;
18 };

Thanks for that addition, @deadlydud! I think this was originally created for version 5.x, and I haven't used PHP much in the last 10 years 😆