PHP - Need Help Assigning Array Key And Value To A String

How can I get rid of  "Notice:Array to string conversion on line 78" error from this code?  can i get a little help please? 

This is line 78 :         $table .= $columnValue;

Basically this code's supposed to do is to group the values from grpa, grpb of datatb to 4 different categories according to their value. and as end result, category index number  of each category is put into the html table.

Thanks.

<?php

 include ("configd.php");

  $sql = "SELECT grpa, grpb From datatb";
  $result = $conn->query($sql);
  
  //categories
  
  $cat = [
    4 => [1, 2, 3, 4],
    3 => [5, 6],
    2 => [7, 8],
    1 => [9, 10]
];
while ($row = $result->fetch_assoc()) {
    foreach ($cat as $catKey => $catValue) {
        $columns[$catKey] = array_filter($row, function ($col) use ($cat, $catKey) {
            if (in_array($col, $cat[$catKey])) {
                return true;
            }
        });
    }

    $output[] = $columns;
}


// the output 

$table = <<<EOT
<table class='maintable'>
    <tr><th>9:10</th></tr>
    <tr><th>7:8</th></tr>
    <tr><th>5:6</th></tr>
    <tr><th>1:4</th></tr>
</table>

EOT;


foreach ($output as $row) {
    $table .= "<table class='maintable'>";

    foreach ($row as $columnKey => $columnValue) {
        $table .= "<tr>";
        $table .= "<td>";
        $table .= $columnValue;
        $table .= "</td>";
    }

    $table .= "</tr>";
    $table .= "</table>";
}




echo $table;

?>

	<?php
    $srs = array();
    for ($section = 1; $section < 5; $section++) {
        for ($row = $section; $row < 10; $row++) {
            for ($seat = $row; $seat < 20; $seat++) {
                $srs[] = array(
                    'section_name' => $section,
                    'row_name' => $row,
                    'seat_name' => $seat);
            }
        }
    }
	
    output(convert_array($srs));
	// Converts the array
    function convert_array($input)
    {
	        return $input;
    }
	    function output($obj)
    {
        echo "<pre>";
        print_r($obj);
        echo "</pre>";
        die;
    }
?>
	

I'm trying to convert those array number to string, add implode() to make it happen, but seem PHP didn't recognize variable in $input. Like this implode("Section: ", $input) or (Section: ", $srs). Can you help me?

Thanks,

Gary