Do not calculate contrast from the stored foreground channels alone when alpha is below 1:
use PhpColor\Color\Color;
use PhpColor\Color\Contrast\ContrastSolver;
$foreground = Color::parse('#000000')->withAlpha(0.54);
$background = Color::parse('#ffffff');
$visible = ContrastSolver::composite($foreground, $background);
$ratio = ContrastSolver::compositedRatio($foreground, $background);
assert('#757575' === $visible->toHex());
assert($ratio >= 4.58);
The ratio applies to this one foreground-background pair. If another translucent layer sits between them, composite the layers in display order first.