The upgrader produces this warning several times:

[phpBB Debug] PHP Warning: in file [ROOT]/upgrader.php on line 154: Illegal string offset 'limit'

The problem is that the check on line 152 no longer works on PHP 5.4:

if (isset($dbms_type_map['mysql_40'][$orig_column_type . ':']['limit'][0]))

The limit element does not exist, but

isset($...['limit'][0])

returns true instead of false. Attempting to access that element throws a warning and returns the first letter of `$orig_column_type . ':'`.