Description:
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After using session_regenerate_id(), the memcache session extension doesn't free the lock associated to the original session_id, so if you want to reuse it you have to wait memcache.lock_timeout seconds to pass.

Test script:
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<!-- begin script1.php -->
<?php
session_id( 'testid' );
session_start();
session_regenerate_id(); // new randomly assigned id



<!-- begin script2.php -->
<?php
session_id( 'testid' );
$t = time();
session_start();
print "Elapsed: ".( $t - time() ); // here will display some value near memcache.lock_timeout

Expected result:
----------------
Elapsed: 0

Actual result:
--------------
Elapsed: 15