Mars_Weight.php

<?php

// Use constants
define('MARS_MULTIPLE', 0.378);

function main() {
    $earth_weight_str = readline('Enter a weight on Earth: ');

    // Get the numeric value since readline() returns a string
    $earth_weight = (float) $earth_weight_str;

    // Having a variable for each piece of information is a good habit
    $mars_weight = $earth_weight * MARS_MULTIPLE;
    $rounded_mars_weight = round($mars_weight, 2);

    // Note the string concatenation!
    echo 'The equivalent weight on Mars: ' . $rounded_mars_weight . PHP_EOL;
}

if (php_sapi_name() === 'cli') {
    main();
}
?>


//--------------------------------------------------------------------------------------------

// Mars Weight Solution
// There are three key stages to solving this problem:

// 1. Getting the Earthling's weight from them, which I need the input function for. 

// 2. Converting the Earthing's weight from a string to a number so I can do math with it. 
// I use the float function to do this, since the weight isn't necessarily a whole number

// 3. Calculating the weight on Mars, which I do by multiplying the Earth weight by 0.378. 
// To make the program easy to read, I store this number in a constant called MARS_MULTIPLE.