A client website just upgraded from PHP4 to PHP5 and MySQL4 to MySQL5 and completely broke. Doing such significant upgrades should have been tested first, but for some reason didn’t happen. I got invited to fix and ran across several problems:
MySQL queries containing some explicit JOINs broke. A simple query like this doesn’t work in MySQL5:
SELECT table1.*, table2.* FROM table1, table2 LEFT JOIN table3 on table1.col1 = table3.col1
In MySQL 5, the JOIN operator now has a higher precedence than the comma operator, so it interprets the query differently. See this post or the MySQL documentation for more information. The quick fix is to put parenthesis around the tables in the FROM statement, like this:
SELECT table1.*, table2.* FROM (table1, table2) LEFT JOIN table3 on table1.col1 = table3.col1
The other significant problem was in the upgrade from PHP4 to PHP5, the XML parsing functions are completely different. PHP 4 used the domxml extentions, where PHP 5 uses a newere DOM extention.
From http://www.php.net/manual/en/ref.domxml.php:
It will, however, never be released with PHP 5, and will only be distributed with PHP 4. If you need DOM XML support with PHP 5 you can use the DOM extension. This domxml extension is not compatible with the DOM extension.
The solution for fixing this, however is quite a bit more complicated. I had to rewrite the XML producing scripts to use the new functionality. Fortunately, the new DOM functionality is pretty straightforward and easier to write, so porting it from one to the other is fairly straightforward, but does require some effort.