A memory-efficient adjacency matrix that can store values in the range 0-15 (4 bits) per cell. Takes roughly 8 * v bytes of memory space (where v = number of vertices). Suitable for Minecraft redstone wires which have a strength of up to 15.

AdjacencyMatrix.php

<?php

final class AdjacencyMatrix{

	public static function read(string $data) : self{
		return new self(array_values(unpack("J*", $data)));
	}

	public static function of(int $size) : self{
		return new self(array_fill(0, ($size >> 4) | $size, 0));
	}

	/**
	 * @param list<int> $values
	 */
	private function __construct(
		private array $values
	){}

	public function set(int $x, int $y, int $value) : void{
		$this->values[($x >> 4) | $y] |= ($value << ($x * 4));
	}

	public function get(int $x, int $y) : int{
		return ($this->values[($x >> 4) | $y] >> ($x * 4)) & 15;
	}

	public function write() : string{
		return pack("J*", ...$this->values);
	}
}

$m = AdjacencyMatrix::of(8);
for($i = 0; $i < 4; $i++){
	$m->set(4, $i, ($i + 1) * 2);
}
for($i = 0; $i < 4; $i++){
	$m->set(6, 6-$i, 15);
}
for($i = 0; $i < 4; $i++){
	var_dump($m->get(4, $i));
}
for($i = 0; $i < 4; $i++){
	var_dump($m->get(6, 6-$i));
}

var_dump($m);
var_dump(AdjacencyMatrix::read($m->write()));