I'm not entirely sure when this started, but it must be in between php 5.3.2 and 5.5.0. Anyway, the thing is php will now output a notice when strtr is called with multidimensional $replace_pairs array, even if the array entries are not used.

And this is precisely what occurs in includes/sessions.php, where format_date will call strtr with one array in the $date_cache[$format]['lang'] entry (the AGO key).

From my experience, the result is not altered, strtr works just as before, but throws a notice on recent versions.

<?php

$str = 'hi all, I said hello';

$replace_pairs = array(

	'all' => 'everybody',

	'unused' => array('somtehing', 'something else'),

	'hello' => 'hey',

);

// php 5.5.0 Notice: Array to string conversion in test.php on line 9

echo strtr($str, $replace_pairs); // hi everybody, I said hey

Replacing strtr with @strtr seems a working solution.