PHP more elegant way to make random 1 or 0 binary pattern

I am wondering if there is a more elegant way to create a random mix of 4 1's and 0's in PHP. The way I do it works but I am curious if there is a way to do the same thing with less code?

$b1 = rand(0,1);
$b2 = rand(0,1);
$b3 = rand(0,1);
$b4 = rand(0,1);

$randpattern = $b1.$b2.$b3.$b4;