PHP: How to get multiple checkbox values from database for same ID?

I would rather go in direction of threating checkboxes as binary flags and store their binary OR operation into DB as integer. It is really easy to implement, and will simplify record relations in your database. To do that you need to change values of checkboxes into 2^n values. For example

<input type="checkbox" name="subcat[]" value="1">Mech
<input type="checkbox" name="subcat[]" value="2">Ele
<input type="checkbox" name="subcat[]" value="4">Civ
<input type="checkbox" name="subcat[]" value="8">Air
<input type="checkbox" name="subcat[]" value="16">BSL

selecting Mech and Civ should be storen in DB as 5 (binary 00101), Air+BSL would be 24 (11000b) and so on. To revert this process you would just simply test every bit of stored value and check/uncheck corresponding boxes in loop. I hope that you will get my point.

EDIT: example code as requested. Be aware that I AM NOT a PHP programmer so there could (and definetly will be) syntax errors but I will do my best To get user input you have to simply add values of checked boxes

$selected=0;
foreach($POST['subcat'] as $checkedBox){
    $selected=$selected+$checkedBox
    }

So now variable $selected will have encoded status of all checkboxes To check which boxes (I never have troubles with that word) should be check or not you have to test every bit of stored value in DB. I dont know how are you generating your view, so i will provide only a template, you have to figure it out how to apply it to your project

$valueFromDb= ///  some value fetched from DB
$maxValue= //// the highest value of your checkboxes, in my eg. 16
for($testValue=1;$testValue<=$limit;$testValue=$testValue << 1){
    $result=($valueFromDb & $testValue) > 0;
    // do something with checked result for current checkbox
}

variable result above will have checked status (boolean value) of coresponding checkboxes. Each loop iteration coresponds to different checkbox in order from Mech to BSL one by one. Alternatively you can print checkboxes directly and set them as checked with if inscruction $valueFromDb= // fetched value

<input type="checkbox" name="subcat[]" value="1" <?php if($valueFromDb & 1 >0) echo('checked') ?php>Mech
<input type="checkbox" name="subcat[]" value="2" <?php if($valueFromDb & 2 >0) echo('checked') ?php>>Ele
<input type="checkbox" name="subcat[]" value="4" <?php if($valueFromDb & 4 >0) echo('checked') ?php>>Civ
 .... and so on and on and on

I have just noticed that combining those 2 methods would result in generating checboxes dynamicly in loop so the code will be much more readable. All you would need is to provide map of checbox value labels:) I hope that eg. above will definetlly clearify how to apply such solution.

<?php
 $checksub = array('1','2','3');

?>

<form action="<?php echo site_url('index/do_upload') ?>" method="post" enctype="multipart/form-data">
    <label>Select file : <input type="file" name="userfile"></label>
    <input type="submit" name="action">
    <input type="checkbox" name="subcat[]" value="1" <?php if (in_array(1,$checksub)){ echo "checked"; }?>/> 

</form>

You will need to use the PHP if statement; I don't really understand your db structure, else I would provide code however, if you use PHP if, for each checkbox, check in the database to see if it's selected then; if it is selected

echo '<input type="checkbox" name="subcat[]" value="Mech" selected>Mech';

else;

echo '<input type="checkbox" name="subcat[]" value="Mech">Mech';

Do you understand what I mean?

Source; HTML/CSS/PHP Developer for 3 years.

You can retrieve the subcats with:

SELECT party_code, GROUP_CONCAT(scid) scids
FROM mst_scadd
GROUP BY party_code

Then when you're fetching the rows from the database, you can do:

$scids = array_flip(explode(',', $row['scids']));

That will create an associative array whose keys are the subcategories. Then your code for displaying the checkboxes can be something like:

foreach ($all_subcats as $sc) {
    echo '<input type="checkbox" name="subcat[]" value="' . $sc . '" ' .
        (isset($scids[$sc]) ? 'checked' : '') . '>' . $sc;
}