HTML:

```
<!DOCTYPE html>
<html>
<head>
<title>Remote Control Boat Speed Calculator</title>
</head>
<body>
<h1>Remote Control Boat Speed Calculator</h1>
<form method="post" action="calculate.php">
<label for="prop_diameter">Propeller Diameter (ft):</label>
<input type="number" id="prop_diameter" name="prop_diameter" step="0.01" required><br>
<label for="prop_pitch">Propeller Pitch (ft):</label>
<input type="number" id="prop_pitch" name="prop_pitch" step="0.01" required><br>
<label for="engine_rpm">Engine RPM:</label>
<input type="number" id="engine_rpm" name="engine_rpm" required><br>
<label for="boat_weight">Boat Weight (lbs):</label>
<input type="number" id="boat_weight" name="boat_weight" required><br>
<button type="submit">Calculate</button>
</form>
</body>
</html>
```

We can add a weight calculator to the Remote Control Boat Speed Calculator to take into account the weight of the boat. The additional calculation would involve factoring in the boat's weight to determine the impact on the boat's speed.

Here's an updated PHP implementation of the Remote Control Boat Speed Calculator with the weight calculation:

PHP:

```
<?php
// Define the constants
define('WHEEL_CIRCUMFERENCE', 6.28); // in feet
define('SECONDS_PER_HOUR', 3600);
// Get the input variables from the user
$propDiameter = $_POST['prop_diameter'];
$propPitch = $_POST['prop_pitch'];
$engineRPM = $_POST['engine_rpm'];
$boatWeight = $_POST['boat_weight'];
// Calculate the wheel RPM
$wheelRPM = ($engineRPM * $propPitch) / (WHEEL_CIRCUMFERENCE * $propDiameter);
// Calculate the boat's speed in MPH
$speed = ($wheelRPM * WHEEL_CIRCUMFERENCE * SECONDS_PER_HOUR) / (5280 * 60);
// Adjust the speed based on the boat's weight
$speed = $speed * sqrt((220 + $boatWeight) / 220);
// Output the results
echo "Boat speed: " . number_format($speed, 2) . " MPH\n";
?>
```

This implementation calculates the speed of a remote control boat based on the propeller diameter, pitch, and engine RPM. It uses the `define`

function to define the constants, and retrieves the input variables from the user using the `$_POST`

superglobal variable. It then performs the calculations using standard PHP operators and outputs the results using the `echo`

function. Note that the output is formatted using the `number_format`

function to display the results with two decimal places.

Note that this implementation assumes that the propeller diameter is measured in feet, and that the propeller pitch is the distance that the propeller would move forward in one revolution, also measured in feet. If these values are provided in a different unit, the formula would need to be adjusted accordingly.

Also, keep in mind that this calculation is an estimation and may not be entirely accurate, as factors such as water conditions, wind speed, and boat weight can affect the actual speed of the boat.

Finally, ensure that the units of measurement for all inputs are consistent with each other.

We've added an additional input variable `$boatWeight`

to retrieve the weight of the boat from the user. We've also included an adjustment to the boat's speed based on its weight, using a formula that takes into account the boat's weight and the square root of the boat's weight plus 220. This formula provides an estimate of how the boat's weight will impact its speed.