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What other informations should I provide in this case? I can tell for sure that the only change on this server was to upgrade the PHP to 5.1.0b2.

Just installed 5.1.0b3 and it looks like there is no more CPU overloading, I will reopen this BUG report if I encounter the same problem with the 5.1.0b3 version.

Please, keep this report bogus until you're able to prove that it was PHP fault and provide more info about it. 

Ok then, I will have to downgrade from PHP 5.0.1b3 to 5.0.1b2 and withouth any change into the scripts the CPU will go back to its overloaded state.
Which I will.

So, I've downgraded PHP from 5.0.1b3 to 5.0.1b2, accessed a page served by this server and the ps -aux command gives me this result after few minutes:
USER       PID %CPU %MEM    ...
apache     5042 77.3  1.3   ...
It is gradually increasing, very fast.
After another 2 minutes the same process shows:
USER       PID %CPU %MEM    ...
apache     5042 81.5  1.3   ...

After using the kill command on the process with the PID 5042 the CPU load starts to decreade gradually, at the same speed it was increasing before.
USER       PID %CPU %MEM    ...
apache     5042 65.4  1.3   ...

And, once again, upgrading to the PHP 5.0.1b3 the CPU overloading problem goes away. Doesn't that mean that we have something going wrong into the 5.0.1b2 version?

In fact, it might be one of the bugs fixed with the 5.0.1b3 release:

- Upgraded bundled SQLite library for PDO:SQLite to 3.2.2 (Ilia)
- Moved extensions to PECL:
  . ext/dbx         (Jani, Derick)
  . ext/ircg        (Jani, Derick)
- Added PDO_MYSQL_ATTR_USE_BUFFERED_QUERY parameter for pdo_mysql. (Ilia)
- Added date_timezone_set() function to set the timezone that the date
  functions will use. (Derick)
- Added pg_fetch_all_columns() function to fetch all values of a column from
  a result cursor. (Ilia)
- Added support for LOCK_EX flag for file_put_contents(). (Ilia)
- Implemented feature request #33452 (Year belonging to ISO week). (Derick)
- Fixed support for shared extensions on AIX. (Dmitry)
- Fixed memory corruption in pg_copy_from() in case the as_null parameter was
  passed. (Derick)
- Fixed crash inside stream_get_line() when length parameter equals 0. (Ilia)
- Fixed bug #33673 (Added detection for partially uploaded files). (Ilia)
- Fixed bug #33605 (substr_compare() crashes with negative offset and length). 
  (Tony)
- Fixed bug #33597 (setcookie() "expires" date format doesn't comply with RFC).
  (Tony)
- Fixed bug #33588 (LDAP: RootDSE query not possible). (Jani)
- Fixed bug #33578 (strtotime() doesn't understand "11 Oct" format). (Derick)
- Fixed bug #33562 (date("") crashes). (Derick)
- Fixed bug #33536 (strtotime() defaults to now even on non time string).
  (Derick)
- Fixed bug #33532 (Different output for strftime() and date()). (Derick)
- Fixed bug #33523 (Memory leak in xmlrpc_encode_request()). (Ilia)
- Fixed bug #33520 (crash if safe_mode is on and session.save_path is changed).
  (Dmitry)
- Fixed bug #33512 (Add missing support for isset()/unset() overloading to
  complement the property get/set methods). (Dmitry)
- Fixed bug #33491 (crash after extending MySQLi internal class). (Tony)
- Fixed bug #33475 (cURL handle is not closed on curl_close(). (Ilia)
- Fixed bug #33469 (Compile error undefined reference to ifx_checkAPI). (Jani)
- Fixed bug #33433 (strtoll not available on Tru64). (Jani, Derick)
- Fixed bug #33415 (strtotime() related bugs). (Derick)
- Fixed bug #32660 (Assignment by reference causes crash when field access is
  overloaded (__get)). (Dmitry)
- Fixed bug #31358 (Older GCC versions do not provide portable va_copy()).
  (Jani)
- Fixed bug #31158 (array_splice on $GLOBALS crashes). (Dmitry)
- Fixed bug #33156 (cygwin version of setitimer doesn't accept ITIMER_PROF).
  (Nuno)
- Fixed bug #30828 (debug_backtrace() reports incorrect class in overridden
  methods). (Dmitry)
- Fixed bug #30519 (Interface not existing says Class not found). (Dmitry)
- Fixed bug #30096 (gmmktime does not return the corrent time). (Derick)
- Fixed bug #30052 (Crash on shutdown after odbc_pconnect()). (Edin)
- Fixed bug #28377 (debug_backtrace is intermittently passing args). (Dmitry)
- Fixed bug #28072 (static array with some constant keys will be incorrectly
  ordered). (Dmitry)
- Fixed bug #27268 (Bad references accentuated by clone). (Dmitry)

Errata:

Read PHP 5.1.0b2 and 5.1.0b3 instead of 5.0.1b2 and 5.0.1b3 above.

My bad, too concerned about the problems encountered.