in my php code i have:
echo "<B>Numero de Base de Datos</B> : ".mysql_list_dbs($sql)."<BR>"; # ERROR                                     
echo "<B>Numero de Tablas en ifrprueba</B> : ".mysql_list_tables("ifrprueba",$sql)."<BR>"; #ERROR  

ifrprueba is a database that exists. The result of these funcions are:

Numero de Base de Datos : 5
Numero de Tablas en ifrprueba : 6

The point is that there are not 5 Databases, but 3. And there are not 6 tables in that database but 1.

bbs:/usr/local/bin # ./mysqlshow -u ifr -p              
Enter password: 
+-----------+
| Databases |
+-----------+
| ifrprueba |
| mysql     |
| test      |
+-----------+
bbs:/usr/local/bin # ./mysqlshow -u ifr -p ifrprueba
Enter password: 
Database: ifrprueba
+--------+
| Tables |
+--------+
| test   |
+--------+

Thank You.

Josafat Moragrega