Need an array-like structure in PHP with minimal memory usage

The most memory efficient you’ll get is probably by storing everything in a string, packed in binary, and use manual indexing to it.

$storage="";

$storage .= pack('l', 42);

// ...

// get 10th entry
$int = current(unpack('l', substr($storage, 9 * 4, 4)));

This can be feasible if the “array” initialisation can be done in one fell swoop and you’re just reading from the structure. If you need a lot of appending to the string, this becomes extremely inefficient. Even this can be done using a resource handle though:

$storage = fopen('php://memory', 'r+');
fwrite($storage, pack('l', 42));
...

This is very efficient. You can then read this buffer back into a variable and use it as string, or you can continue to work with the resource and fseek.