PHP assign values within in_array

Question

What is a more efficient way of writing this sample code? Its purpose is to create a variable for days of the week based on a numerical value. This data is obtained from user input checkboxes. If the user doesn't select any options, they will be assigned the Online value. Fiddle here - https://3v4l.org/Sghnh#v7.4.30

 <?php
$classDays = [1,3,5];
if($classDays[0] == '') {
 $classDaysD = 'Online';
} else {
if (in_array(1, $classDays)) {
 $classDaysD = 'M';
}
if (in_array(2,$classDays)) {
 $classDaysD .= ' T';
} 
if (in_array(3,$classDays)) {
 $classDaysD .= ' W';
} 
if (in_array(4,$classDays)) {
 $classDaysD .= ' Th';
} 
if (in_array(5,$classDays)) {
 $classDaysD .= ' F';
} 
if (in_array(6,$classDays)) {
 $classDaysD .= ' S';
} 
if (in_array(7,$classDays)) {
 $classDaysD .= ' Su';
} 
if ($classDaysD == '') {
 $classDaysD = 'M & W';
}
}
echo $classDaysD;

Here is the checkbox section of the form, viewable here http://jsfiddle.net/S9qrZ/1/:

Which days of the week are your classes for the syllabi you are about to create?

<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="1">
 <label class="form-check-label" for="classDays[]">Mon</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="2">
 <label class="form-check-label" for="classDays[]">Tues</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="3">
 <label class="form-check-label" for="classDays[]">Wed</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="4">
 <label class="form-check-label" for="classDays[]">Thur</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="5">
 <label class="form-check-label" for="classDays[]">Fri</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="6">
 <label class="form-check-label" for="classDays[]">Sat</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="7">
 <label class="form-check-label" for="classDays[]">Sun</label>
</div>
<div class="form-check form-check-inline">
 <input class="form-check-input" type="checkbox" id="classDays[]" name="classDays[]" value="Online" checked>
 <label class="form-check-label" for="classDays[]">Online (uncheck if face to face class)</label>
</div>

• KIKO Software: You code generates a warning for a seemingly valid value of $classDays, perhaps you should solve this problem first? It would also be helpful if you gave a better description of what the code is used for and what it is suppose to accomplish. We can guess, but should we? Why should $classDaysD = '' result in 'M & W' and $classDaysD = [''] in 'Online'? That seems very arbitrary.
• Sᴀᴍ Onᴇᴌᴀ: Also, what is meant by "more efficient way"? Is that in terms of memory and/or processing power, lines of code, or some other measure?
• Tim M: @KIKOSoftware - 3v4l.org/Sghnh#v7.4.30 is the link I posted and there isn't an error. The link you posted is missing the values for $classDays.
• Tim M: @SᴀᴍOnᴇᴌᴀ - You've summed it up :) Primarily, my code seems repetitive so I would think it would be better to loop, but I'm not sure of the best way to go about that.
• KIKO Software: I agree, I made a mistake. I accidentally mixed $classDays and $classDaysD. This is due to the small difference between these names. Sorry, for that. Still, I would like to know what input and output you expect. How can this expression: ($classDaysD == ''), ever be true? When should it be true?

Answer 1

I can't get clear answers to my question in the comments, but I think I can try something. It would seem logical to define an array that contains the days of the week, like this:

const WEEKDAYS = ['M', 'T', 'W', 'Th', 'F', 'S', 'Su'];

I have to use an array because you don't use 2 or 3 letters, but a mixture of 1 and 2 letter, abbreviations for the days of the week.

Now, instead of using day numbers in the checkboxes, and then translate this to alphabetical week days, I would just use alphabetical week days in the checkboxes, like this:

foreach (WEEKDAYS as $weekdayKey => $weekday) {
 echo '<div class="form-check form-check-inline">' . PHP_EOL .
 ' <input class="form-check-input" type="checkbox" id="classDays' . $weekdayKey . '" name="classDays[]" value="' . $weekday . '">' . PHP_EOL .
 ' <label class="form-check-label" for="classDays[]">' . PHP_EOL .
 ' ' . date('l', strtotime('2022-01-0' . ($weekdayKey + 3))) . PHP_EOL . 
 ' </label>' . PHP_EOL . 
 '</div>' . PHP_EOL;
}

See: https://3v4l.org/armtj#v7.4.30

Note that date('l', strtotime('2022-01-0' . ($weekdayKey + 3))) just gets the day of the week, in your local language, based on $weekdayKey from 0 to 6.

Now when I process the form, I already have, more or less, the correct data. But validating user input is always important. This processing could look something like this:

if (isset($_POST['classDays']) {
 $classDaysD = implode(' ', array_intersect($_POST['classDays'], WEEKDAYS));
} else {
 $classDaysD = 'Online';
}

The usage of array_intersect() is there to make sure that only values from WEEKDAYS are accepted.

I would strongly advice you to choose another name for the $classDaysD variable. Something like $classDays would do, in my code.

I don't know how to incorporate the 'M & W' result.