Avoid reading too many values from memcahce. Each get() consumes some memory. You should cache the values instead, once they are read. Illustration of bad practice:
<?php // reading from memcache leaks
$m = new Memcache();
$m -> connect( "localhost", "11211" );
$m -> add("foo", "bar");
for(;;) { // this "endless" loop will finish because it eats up all the memory
$res = $m -> get("foo");
print memory_get_usage()."\n"; // this writes bigger and bigger numbers
}