Recently one of our Customers has updated php and mysql version from 4.x to 5.x in the hosting server of their Joomla based website.
After doing this upgration/migration, many pages were not working. They started giving error messages such as, "Deprecated: Function eregi() is deprecated..."
We resolved these issues by replacing deprecated functions with equivalent functions.
For example, replaced split() with explode(). And, replaced eregi() with preg_match('/regexp/i',,).
Note that we should select appropriate modifier in regular expression in the preg_match argument. i-e The modifier should not be the part of the regular expression statement. We need to use rarely used character as modifier. For example we can use "~" as modifier.
This blog post was very helpful for resolving many issues.
I think Joomla is used by many people. But still I couldn't find any easy way to update the Joomla code automatically according to php/mysql upgration.
If you know any good article for doing php/mysql up-gradation you can mention it thro' comments. And, you can share your up-gradation experience.