retrieve the id PDO if a value already exists - php

Question

I have two tables which are linked with many many relationship,Normally I work with Mysqli for querying,but now for some reasons I have to use PDO,So my problem is 1st to check if a value doesn't exists, to add it in a table tag,and if not to take the id of the value and add the related data in the join data.
tag-tagmap-post a tag can belong to many posts,and a post can have many tags I try this but when a tag name already exists I can't retrieve the id of the tag.
$sql = "
INSERT INTO tag (name)
SELECT * FROM (SELECT :name) AS tmp
WHERE NOT EXISTS (
SELECT name FROM tag WHERE name =:name
) LIMIT 1";
try {
$db = getConnection();
$stmt = $db->prepare($sql);
$stmt->bindParam(":name", $post->name);
$stmt->execute();
$test=$post->id = $db->lastInsertId();
$db = null;
//echo json_encode($post);
} catch(PDOException $e) {
//error_log($e->getMessage(), 3, '/tmp/php.log');
echo '{"error":{"text":'. $e->getMessage() .'}}';
}
$sql2="INSERT INTO tagmap (tag_id,post_id,user_id) VALUES(:id2,:post_id,:id)";//
try {
$db = getConnection();
$stmt = $db->prepare($sql2);
$stmt->bindParam("id2", $test);//tag id
$stmt->bindParam("post_id", $post->post_id);//post_id
$stmt->bindParam("id", $id);
$stmt->execute();
//$post2->id = $db->lastInsertId();
$db = null;
echo json_encode(array("result"=>$test));
} catch(PDOException $e) {
//error_log($e->getMessage(), 3, '/tmp/php.log');
echo '{"error":{"text":'. $e->getMessage() .'}}';
}
Thank you for your help!!!

Answer 1

As documented under PDO::prepare():
You cannot use a named parameter marker of the same name more than once in a prepared statement, unless emulation mode is on.
That said, the insertion query itself is over-complicated (and contains syntax errors: there is no LIMIT clause in the INSERT syntax). You could instead:
define a UNIQUE key over (name) in the tag table:
ALTER TABLE tag ADD UNIQUE (name);
use INSERT ... ON DUPLICATE KEY UPDATE:
INSERT INTO tag (name) VALUES (?)
ON DUPLICATE KEY UPDATE id = LAST_INSERT_ID(id);
This will ensure that $db->lastInsertId() returns the ID of the relevant record whether it has been newly inserted or it already existed.