You can simply make a floor with decimals function by using the round() function and subtract 0.05 to the value if the value must have 1 decimal accuracy, or 0.005 for 2 decimals accuracy, or 0.0005 for 3 decimals etc.
Example :
function floorDec($input, $decimals)
{
return round($input - (5 / pow(10, $decimals + 1)), $decimals);
}