PHP how to extract keys names and values from associative array for mysql query

You can implode an associative array, but you cannot implode a multi-dimensional array. That's what the error is telling you. For example...

$my_array = array('a'=>'1', 'b'=>'2', 'c'=>'3');
echo "\n\n".implode(',',array_keys($my_array));
echo "\n\n".implode(',',$my_array)."\n\n";

Results in...

a,b,c

1,2,3

But...

$my_array = array(
    array('a'=>'1', 'b'=>'2', 'c'=>'3'),
    array('d'=>'4', 'e'=>'5', 'f'=>'6')
);
echo "\n\n".implode(',',array_keys($my_array));
echo "\n\n".implode(',',$my_array)."\n\n";

results in...

0,1
PHP Notice:  Array to string conversion in /test.php on line 9

Fixing your code means dealing with the individual data elements. Echo'd out they'd look like this:

selected, user_id, user_name, user_company, user_email
,3,nome3,azien3,email3
1,6,nome6,azien6,email6

So, the basic code would look something like...

$fields = implode(",", array_keys($Arr));
echo $fields."\n";
foreach($Arr as $key=>$val){
    $newdata = "'" . implode("','", $Arr[$key]) . "'";
    echo $newdata."\n";
}

And a PDO INSERT statement would be built like this...

$my_keys = array_keys($Arr[0]);

$stmt=$pdo->prepare("DESC my_table");
$stmt->execute();
$whitelist_columns=$stmt->fetchAll(PDO::FETCH_COLUMN);
foreach($my_keys as $key){
    if(!array_search($key,$whitelist_columns)){ echo "ERROR!"; }
}

$field_names = implode(",", $my_keys);       // build column list
$q_markers = implode(",", $my_keys);     // build PDO value markers

$stmt = $pdo->prepare("INSERT INTO my_table (".$field_names.") VALUES (".$q_markers.")");
foreach($Arr as $key => $val){
    foreach($val as $bind_marker => $bind_val){
        $stmt->bindParam($bind_marker, $bind_val);
    }
    $stmt->execute();
}

Note the section of code with the whitelist variables. The purpose of that code is to protect against SQL injection due to creating the query with unbound column references. PDO does not allow you to bind column names in the same way it does cell data. To protect yourself you must prove that the incoming data matches the columns in the table. If they don't, do something about it (echo "ERROR";). Usually you want to stop that INSERT completely and log the issue somewhere.

$my_keys = array_keys($Arr[0]);
$q_marks = array();

$stmt=$pdo->prepare("DESC my_table");
$stmt->execute();
$whitelist_columns=$stmt->fetchAll(PDO::FETCH_COLUMN);
foreach($my_keys as $key){
    if(!array_search($key,$whitelist_columns)){ echo "ERROR!"; }
    array_push($q_marks, "?");
}

$field_names = implode(",", $my_keys);       // build column list
$field_markers = implode(",", $q_marks);

$stmt = $pdo->prepare("INSERT INTO my_table (".$field_names.") VALUES (".$field_markers.")");

foreach($Arr as $key => $val){
    $stmt->execute($val);
}

The above code is an example of using PDO without bindParam or bindValue. It comes with a price, though usually there's no actual cost. bindParam and bindValue allow you to specifically identify the data type. E.G., bindParam('myval', $myval, PDO::PARAM_INT). When variables are passed as above, you can't do this. Most of the time this is a non-issue as PHP correctly identifies the data type. When PHP does become confused (or if you simply want to impose a check that the data is what you were expecting), then you must use bindParam or bindValue.