Simple implementation of LRU caching in PHP
This implementation is quite simple. I compared it with some others in PHP as well as other languages like C++, Java and JavaScript. Some use a node class to store a key and value separately (e.g. this PHP implementation) but for a simple implementation your approach seems fine. 

I noticed that the method isFull() only explicitly returns a value when true is returned. While it works because the conditional that calls that method would this implicitly evaluate the empty return to a false-y  value, some would argue that methods that return a value should do so in every case, for the sake of readability. That could be achieved by simply returning the conditional:

private function isFull()
{
    return count($this-&gt;cache) &gt;= $this-&gt;size);
}


And one other simplification would be to set the property/instance variable $cache to an empty array when it is declared, since it can be evaluated at compile time and does not depend on any run-time information to be evaluated, instead of inside the constructor:

private $cache = [];