I'm trying to do it like this:

Every single user can choose fields (like structures on MySQL) where this fields can handle their respective value, it's like doing a DB inside a DB.  

But how can I do it using a single table?  

(not talking about user accounts etc where I should be able to use a pointer to his own "structure")  

Do something like: varchar Key where register something like "Name:asd" where PHP explode : to get the respective structure ('name' in this case) and the respective value? ('asd')
Use BLOB? can someone turn the light on for me? I don't know how to do something where works better than my current explanation...

I know my text is confuse and sorry for any bad english.

EDIT:  

Also, they could add multiple keys/"structures" where accepts a new value  

And they are not able to see the Database or Tables, they still normal users  

My server does not support Postogre
['https://Stackoverflow.com/questions/11306060', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/996134/']
Don't start building a database in a database. In this case, since the user makes the field by himself there is no relation between the fields as I understand? In that case it would make sense to take a look at the NoSQL databases since they seem to fit very good for this kind of situations. 

Another thing to check is something like:
<http://www.postgresql.org/docs/8.4/static/hstore.html>

Do not try to build tables like: records, fields, field types etc. That's a bad practice and should not be needed.

For a more specific answer on your wishes we need a bit more info about the data the user is storing.
Use a table with key-value pairs. So three columns:

* user id
* key ("name")
* value ("asd")

Add an index on *user id*, so that you can query a user's attributes easily. If you wanted to query all users with the same properties, then you could add a second index on *key* and/or *value*.