sed replace single/double quoted text?

Having some difficulty in replacing some single/double quoted text with sed and was wondering what's the correct method for these 2 examples

to change Memached.ini file contents from

[server]
server[] = "localhost:11211"

to

[server]
server[] = "localhost:11211"
server[] = "localhost:11212"

and to change memcache.php file contents for these lines from

define('ADMIN_USERNAME','username');    // Admin Username
define('ADMIN_PASSWORD','password');    // Admin Password

$MEMCACHE_SERVERS[] = 'mymemcache-server1:11211'; // add more as an array
$MEMCACHE_SERVERS[] = 'mymemcache-server2:11211'; // add more as an array

to

define('ADMIN_USERNAME','myusername');  // Admin Username
define('ADMIN_PASSWORD','mypassword');      // Admin Password

$MEMCACHE_SERVERS[] = 'localhost:11211'; // add more as an array
$MEMCACHE_SERVERS[] = 'localhost:11212'; // add more as an array

I tried for example

sed -i 's/'ADMIN_USERNAME','memcache'/'ADMIN_USERNAME','u'/g' /var/www/html/memcache.php

while command runs, memcache.php file isn't changed at all ?