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

A PHP Judy Array will use significantly less memory than a standard PHP array, and an SplFixedArray.

I quote "An array with 1 million entries using regular PHP array data structure takes 200MB. SplFixedArray uses around 90 megabytes. Judy uses 8 megs. Tradeoff is in performance, Judy takes about double the time of regular php array implementation."

You could use an object if possible. These often use less memory than array's. Also SplFixedArray is an good option.

But it really depends on the implementation that you need to do. If you need an function to return an array and are using PHP 5.5. You could use the generator yield to stream the array back.

You can try to use a SplFixedArray, it's faster and take less memory (the doc comment say ~30% less). Test here and here.

Use a string - that's what I'd do. Store it in a string on fixed offsets (16 or 20 digits should do it I guess?) and use substr to get the one needed. Blazing fast write / read, super easy, and 600.000 integers will only take ~12M to store.

base_convert() - if you need something more compact but with minimum effort, convert your integers to base-36 instead of base-10; in this case, a 14-digit number would be stored in 9 alphanumeric characters. You'll need to make 2 pieces of 64-bit ints, but I'm sure that's not a problem. (I'd split them to 9-digit chunks where conversion gives you a 6-char version.)

pack()/unpack() - binary packing is the same thing with a bit more efficiency. Use it if nothing else works; split your numbers to make them fit to two 32-bit pieces.

600K is a lot of elements. If you are open to alternative methods, I personally would use a database for that. Then use standard sql/nosql select syntax to pull things out. Perhaps memcache or redis if you have an easy host for that, such as garantiadata.com. Maybe APC.

I took the answer by @deceze and wrapped it in a class that can handle 32-bit integers. It is append-only, but you can still use it as a simple, memory-optimized PHP Array, Queue, or Heap. AppendItem and ItemAt are both O(1), and it has no memory overhead. I added currentPosition/currentSize to avoid unnecessary fseek function calls. If you need to cap memory usage and switch to a temporary file automatically, use php://temp instead.

class MemoryOptimizedArray
{
    private $_storage;
    private $_currentPosition;
    private $_currentSize;
    const BYTES_PER_ENTRY = 4;
    function __construct()
    {
        $this->_storage = fopen('php://memory', 'rw+');
        $this->_currentPosition = 0;
        $this->_currentSize = 0;
    }
    function __destruct()
    {
        fclose($this->_storage);
    }
    function AppendItem($value)
    {
        if($this->_currentPosition != $this->_currentSize)
        {
            fseek($this->_storage, SEEK_END);
        }
        fwrite($this->_storage, pack('l', $value));
        $this->_currentSize += self::BYTES_PER_ENTRY;
        $this->_currentPosition = $this->_currentSize;
    }
    function ItemAt($index)
    {
        $itemPosition = $index * self::BYTES_PER_ENTRY;
        if($this->_currentPosition != $itemPosition)
        {
            fseek($this->_storage, $itemPosition);
        }
        $binaryData = fread($this->_storage, self::BYTES_PER_ENTRY);
        $this->_currentPosition = $itemPosition + self::BYTES_PER_ENTRY;
        $unpackedElements = unpack('l', $binaryData);
        return $unpackedElements[1];
    }
}

$arr = new MemoryOptimizedArray();
for($i = 0; $i < 3; $i++)
{
    $v = rand(-2000000000,2000000000);
    $arr->AddToEnd($v);
    print("added $v\n");
}
for($i = 0; $i < 3; $i++)
{
    print($arr->ItemAt($i)."\n");
}
for($i = 2; $i >=0; $i--)
{
    print($arr->ItemAt($i)."\n");
}

Depending on how you are generate the integers, you could potentially use PHP's generators, assuming you are traversing the array and doing something with individual values.