Dealing with blank fields in an input form with PHP PDO object and postgresql

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PHP

PostgreSQL

PDO

I have a form where I expect many blank input text boxes (standard html)
I need to insert records, with the INSERT statement ignoring those fields which do not have input values. For context, I take the POST variables in the usual way:

if ($_SERVER['REQUEST_METHOD']=="POST") {
        //$well_no = $_POST['well_no'];
        $easting = $_POST['easting'];
        $northing = $_POST['northing'];

etc etc for 15 variables. $well_no is not a POST variable, but computed from 2 other POST variables:

$well_no = $date_completed."-".substr($country,0,2)."-".mt_rand(1,9).mt_rand(10,50);

What I have tried:

I have tried the following approach:

<pre>$fields = [ 'easting', 'northing', 'purpose', 'country', 'admin1', 'admin2', 'admin3', 'settlement',
	'orig_wellno', 'date_completed', 'coord_sys', 'elev', 'status'];

$col = []; // array of columns
$input = []; // array of prepared query input values

// add the well no
$input[] = $well_no; // my computed (not POST) variable
$col = ["`well_no`"];

        
        // loop to get array of field names and values for each
foreach($fields as $field)
{
	// note: empty considers 0 or '0' to be an empty value. this will prohibit a numerical zero value being used.
	//  instead test if the value is or is not an empty string.
	if($_POST[$field] !== '')
	{
		$col[] = "`$field`";
		$input[] = $_POST[$field];
	}
} 


        try {
            
            sql = "INSERT INTO well_parent (".implode(',',$col).") VALUES (".implode(',',array_fill(0,count($col),implode(',', $input))).")";
            $stmnt = $pdo->prepare($sql);
            $user_data = [':well_no'=>$well_no, ':easting'=>$easting, ':northing'=>$northing, ':purpose'=>$purpose,  ':country'=>$country, 
            ':admin1'=>$admin1, ':admin2'=> $admin2, ':admin3'=>$admin3, ':settlement'=>$settlement, ':orig_wellno'=>$orig_wellno, 
            ':date_completed'=> $date_completed, ':coord_sys'=> $coord_sys, ':elev'=>$elev, ':status'=>$status];
            
	echo '<pre>'; print_r($user_data); echo '

';

echo $sql;


$stmnt->execute($user_data);




Here is my output when I print _r the $user_data array:

Array
(
    [:well_no] => 2017-05-09-Ug-922
    [:easting] => 
    [:northing] => 
    [:purpose] => production
    [:country] => Uganda
    [:admin1] => mbale
    [:admin2] => nomu
    [:admin3] => booboo
    [:settlement] => lop
    [:orig_wellno] => bh1
    [:date_completed] => 2017-05-09
    [:coord_sys] => 
    [:elev] => 200
    [:status] => 1
)

So the $user-data array looks good. I also echo the sql statement:

INSERT INTO well_parent (`well_no`,`purpose`,`country`,`admin1`,`admin2`,`admin3`,`settlement`,`orig_wellno`,`date_completed`,`elev`,`status`) VALUES (2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1,2017-05-09-Ug-538,production,Uganda,mbale,nomu,booboo,lop,bh1,2017-05-09,200,1)Error: SQLSTATE[HY093]: Invalid parameter number: :well_no

I get the correct values in the values clause, but repeated 11 times! (Notice the warning I believe the SQL error mentions :well_no because it is the first problem parameter it encounters. I know the problem lies within the array_fill function, and using impode() as the values parameter, yet I'm lost on how to fix this