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/']
In my opinion you should create two tables.

1. with the user info
2. with 3 fields (userid, key and value)

Each user has 1 record in the first table. Each user can have 0 or more records in the second table. This will ensure you can still search the data and that users can easily add more key/value pairs when needed.
While i think the rational answer to this question is the one given by PeeHaa, if you really want the data to fit into one table you could try saving a serialized PHP array in one of the fields. Check out [serialize](http://php.net/manual/en/function.serialize.php) and [unserialize](http://www.php.net/manual/en/function.unserialize.php)

> 
> Generates a storable representation of a value
> 
> 
> This is useful for storing or passing PHP values around without losing
>  their type and structure.
> 
> 
> 

This method is discouraged as it is not at all scalable.