Trailing Comma

The trailing comma is the possibility to leave the last element of a list empty. This is for presentation reasons. In particular, VCS will be able to treat the last line just like any another previous line, with a final comma. This reduces de amount of diff.

Trailing commas were introduced progressively in PHP. In PHP 7.2, in use statements; in PHP 7.3, in method calls; in PHP 8.0, in method declarations; in very old version, in array declarations.

<?php

$a = [1,
      2,
      3,
      // Adding a new element in this array only creates
      // a one line diff, when using the trailing comma
     ];

?>

Documentation

See also How I learned to stop worrying and love the trailing comma.

Related : Comma, Convention