get $_POST name and value in php foreach loop to build a long string

I don't know why you want to insert using SELECT, as this construct is to insert from already existing table data. You can, however, insert multiple VALUES with a single statement. You'd need to do :

// you should filter out values from your $_POST...
$ignoredFields = array('submit', ...);
$fields = array_intersect_key($_POST, array_flip($ignoredFields));

$values = array();
foreach ($fields as $key => $value) {
   $key = mysql_real_escape_string($key);
   $value = mysql_real_escape_string($value);
   $values[] =  "'{$key}', '{$value}'";
}

// creation the insert string
$query = 'INSERT INTO `tabke` (`key`,`value`) VALUES ('.implode(,'),(', $values).')';
$result = mysql_query($query);

** Note ** : I suppose that your table tabke look something like

+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment | 
| key     | varchar(64)      | NO   |     | NULL    |                | 
| value   | text             | NO   |     | NULL    |                | 
+---------+------------------+------+-----+---------+----------------+

As long as your query does not extend max_allowed_packet, this will work just fine. In case your data exceed that size, you can simply use array_chunk and iterate through the chunks and building the INSERT with each chunks.