Update
Since PHP 7 you can accomplish this with the null coalescing operator (keeping in mind that it will bypass existing array element with null value, the same way isset would):
return $table[$key] ?? null;Old answer
First of all, arrays are not implemented as a B-tree, it's a hash table; an array of buckets (indexed via a hash function), each with a linked list of actual values (in case of hash collisions). This means that lookup times depend on how well the hash function has "spread" the values across the buckets, i.e. the number of hash collisions is an important factor.
Technically, this statement is the most correct:
return array_key_exists($key, $table) ? $table[$key] : null;This introduces a function call and is therefore much slower than the optimized isset(). How much? ~2e3 times slower.
Next up is using a reference to avoid the second lookup:
$tmp = &$lookup_table[$key];return isset($tmp) ? $tmp : null;Unfortunately, this modifies the original $lookup_table array if the item does not exist, because references are always made valid by PHP.
That leaves the following method, which is much like your own:
return isset($lookup_table[$key]) ? $lookup_table[$key] : null;Besides not having the side effect of references, it's also faster in runtime, even when performing the lookup twice.
You could look into dividing your arrays into smaller pieces as one way to mitigate long lookup times.
