mysql: using comma-separated values
I would overthink your design and would change to something like this:

Usertable

+--------+----------+
| UserID | Username |
+--------+----------+
|    1   |   PAUL   |
+--------+----------+
|    2   |   Eric   |
+--------+----------+


Servertable

+------------+------------------+
|  ServerID  |    Servername    |
+------------+------------------+
|     1      |      Apache      |
+------------+------------------+
|     2      |      Nginx       |
+------------+------------------+


Serverbooking

+-------------+-----------------+---------------+-------------+
|   ServerID  |     UserID      |      Start    |     End     |
+-------------+-----------------+---------------+-------------+
|      1      |        1        |   somedate    |   somedate  |
+-------------+-----------------+---------------+-------------+
|      2      |        1        |   somedate    |   somedate  |
+-------------+-----------------+---------------+-------------+
|      1      |        2        |   somedate    |   somedate  |
+-------------+-----------------+---------------+-------------+