Let's assume I need to store some data of unknown amount within a database table. I don't want to create extra tables, because this will take more time to get the data. The amount of data can be different.
My initial thought was to store data in a key1=value1;key2=value2;key3=value3 format, but the problem here is that some value can contain ; in its body. What is the best separator in this case? What other methods can I use to be able to store various data in a single row?
The example content of the row is like data=2012-05-14 20:07:45;text=This is a comment, but what if I contain a semicolon?;last_id=123456 from which I can then get through PHP an array with corresponding keys and values after correctly exploding row text with a seperator.
First of all: You never ever store more than one information in only one field, if you need to access them separately or search by one of them. This has been discussed here quite a few times.
Assuming you allwas want to access the complete collection of information at once, I recommend to use the native serialization format of your development environment: e.g. if it is PHP, use serialze().
If it is cross-plattform, JSON might be a way to go: Good JSON encoding/decoding libraries exist for something like all environments out there. The same is true for XML, but int his context the textual overhead of XML is going to bite a bit.
On a sidenote: Are you sure, that storing the data in additional tables is slower? You might want to benchmark that before finally deciding.
Edit:
After reading, that you use PHP: If you don't want to put it in a table, stick with serialize() / unserialize() and a MEDIUMTEXT field, this works perfectly, I do it all the time.