Using the Installer script, configuring a Memcached server results in a broken install. $wgMemCachedServers is configured as such by the installer:
$wgMemCachedServers = array( 0 => "127.0.0.1:11211" );
The documentation, however, specifies that the syntax for a single Memcached server should be:
$wgMemCachedServers = array( "127.0.0.1:11211" );
The first configuration results in an error, as such:
Warning: Unknown: open(tcp://127.0.0.1:11211/sess_(redacted), O_RDWR) failed: No such file or directory (2) in Unknown on line 0
Changing the value to the second(documented) syntax solves the issue. I am unsure of why this matters to PHP, as my understanding of the array() function says that both should produce an equivalent PHP array, but I have not looked further into this behavior.