How do I sort an array by string length then by value in PHP?

Incorporate both checks into your comparator:

function lensort($a,$b){
    $la = strlen( $a); $lb = strlen( $b);
    if( $la == $lb) {
        return strcmp( $a, $b);
    }
    return $la - $lb;
}

You can see from this demo that this prints:

Array
(
    [0] => A
    [1] => B
    [2] => C
    [3] => AA
    [4] => BB
    [5] => BC
)