This is an interesting question. Here's my naive attempt at a solution that's pretty mechanical, but attempts to be fairly uniform. I'm sure it could be improved/optimised significantly.
Edit
I wasn't satisfied with my previous answer (I rushed it so there were some errors - thanks for pointing those out!). I decided to remove that and attempt a different approach, which goes a bit crazy with regexes and filter_input to determine if user input is valid - and if so - cast it to the appropriate type.
This makes validating the rating against the chosen scale, (by type and value comparison) a lot more uniform. I sanity-checked this a bit and I think I am happier with this approach ;)
Once again... Assuming a HTML form like:
<form method="post">
<label>rating</label>
<input name="rating" type="text" autofocus>
<label>out of</label>
<select name="scale">
<option value="100">100</option>
<option value="10">10</option>
<option value="6">6</option>
<option value="5">5</option>
<option value="4">4</option>
<option value="A">A</option>
<option value="A+">A+</option>
</select>
<input type="submit">
</form>
And the following PHP to user-submitted input:
<?php
const MAX_STARS = 5;
const REGEX_RATING = '/^(?<char>[a-fA-F]{1}[-+]?)$|^(?<digit>[1-9]?[0-9](\.\d+)?|100)$/';
const REGEX_SCALE = '/^(?<char>A\+?)$|^(?<digit>100|10|6|5|4)$/';
$letters = [
'F-', 'F', 'F+',
'G-', 'G', 'G+',
'D-', 'D', 'D+',
'C-', 'C', 'C+',
'B-', 'B', 'B+',
'A-', 'A', 'A+',
];
if ('POST' === $_SERVER['REQUEST_METHOD']) {
// validate user-submitted `rating`
$rating = filter_input(INPUT_POST, 'rating', FILTER_CALLBACK, [
'options' => function($input) {
if (preg_match(REGEX_RATING, $input, $matches)) {
return isset($matches['digit'])
? (float) $matches['digit']
: strtoupper($matches['char']);
}
return false; // no match on regex
},
]);
// validate user-submitted `scale`
$scale = filter_input(INPUT_POST, 'scale', FILTER_CALLBACK, [
'options' => function($input) {
if (preg_match(REGEX_SCALE, $input, $matches)) {
return isset($matches['digit'])
? (float) $matches['digit']
: strtoupper($matches['char']);
}
return false; // no match on regex
}
]);
// if a valid letter rating, convert to calculable values
if (in_array($scale, ['A+', 'A']) && in_array($rating, $letters)) {
$scale = array_search($scale, $letters);
$rating = array_search($rating, $letters);
}
// error! types don't match
if (gettype($rating) !== gettype($scale)) {
$error = 'rating %s and scale %s do not match';
exit(sprintf($error, $_POST['rating'], $_POST['scale']));
}
// error! rating is higher than scale
if ($rating > $scale) {
$error = 'rating %s is greater than scale %s';
exit(sprintf($error, $_POST['rating'], $_POST['scale']));
}
// done! print our rating...
$stars = round(($rating / $scale) * MAX_STARS, 2);
printf('%s stars out of %s (rating: %s scale: %s)', $stars, MAX_STARS, $_POST['rating'], $_POST['scale']);
}
?>
It's probably worth explaining what the hell is going on with the regexes and callbacks ;)
For example, take the following regex:
/^(?<char>A\+?)$|^(?<digit>100|10|6|5|4)$/'
This regex defines two named subpatterns. One, named <char>, captures A and A+; the other, named <digit> captures 100, 10, 6 etc.
preg_match() returns 0 if there is no match (or false on error) so we can return false in that case, because this means the user input (or the scale) POSTed was not valid.
Otherwise, the $match array will contain any captured values, with char and (optionally) digit as keys. If the digit key exists, we know the match is a digit and we can cast it to a float and return it. Otherwise, we must have matched on a char, so we can strtoupper() that value and return it:
return isset($matches['digit'])
? (float) $matches['digit']
: strtoupper($matches['char']);
Both callbacks are identical (apart from the regexes themselves) so you could create a callable there and maybe save some duplication.
I hope it's not starting to feel a bit convoluted at this stage! Hope this helps :)
