Aug 122008
A deadlock is a situation wherein two or more competing actions are waiting for the other to finish, and thus neither ever does. It is often seen in a paradox like ‘the chicken or the egg‘.
WikiPage : http://en.wikipedia.org/wiki/Deadlock
deadlock_test1.php
<?php $fp1 = fopen("foo1.txt", "w"); if (flock($fp1, LOCK_EX)) { print "Got lock for foo1.txt!n"; sleep(10); $fp2 = fopen("foo2.txt", "w"); if (flock($fp2, LOCK_EX)) { print "Got lock foo2.txt!n"; sleep(10); } flock($fp2, LOCK_UN); flock($fp1, LOCK_UN); } ?>
deadlock_test2.php
<?php $fp1 = fopen("foo2.txt", "w"); if (flock($fp1, LOCK_EX)) { print "Got lock foo2.txt!n"; sleep(10); $fp2 = fopen("foo1.txt", "w"); if (flock($fp2, LOCK_EX)) { print "Got lock foo1.txt!n"; sleep(10); } flock($fp2, LOCK_UN); flock($fp1, LOCK_UN); } ?>
Run deadlock_test1.php and deadlock_test2.php in 2 different shell at a same time to get dead lock. 
Nice .. so If a person wants to kill a server he gotta run this !!!
Modern production servers have advanced deadlock protection, so if you are looking to kill a production engine, tough luck.
They do it by one of these
1. Mutual exclusion Only one process can use a resource at
a time
2. Hold-and-wait A process continues to hold a resource
while waiting for other resources
3. No preemption No resource can forcibly be removed from
a process holding it
4. Circular Wait There is a circular chain of processes where
each holds a resource that is needed by the next in the
circle
Still, its a good example as not many are designed for deadlock prevention !