使用数组执行PDO返回null

Question

I'm trying to use good PDO as always and almost everything works but one query:

$primary = 'my_id';
$table = 'my_table';
// This or...
$statement = $this->conn->prepare("SELECT MAX(:id) AS id FROM :table");
        $statement->bindParam(':id', $primary, PDO::PARAM_STR);
        $statement->bindParam(':table', $table, PDO::PARAM_STR);
        $statement->setFetchMode(PDO::FETCH_ASSOC);
        $statement->execute();

// This one. Both doesn't work.

$statement = $this->conn->prepare("SELECT MAX(:id) AS id FROM :table");
        $statement->setFetchMode(PDO::FETCH_ASSOC);
$arr = array(
            ':id' => 'my_id',
            ':table' => 'my_table',
        );
        $statement->execute($arr);

These just return a null array. I feel so confused. So I have tried that:

$statement = $this->conn->prepare("SELECT MAX(".$primary.") AS id FROM ".$table);
        $statement->setFetchMode(PDO::FETCH_ASSOC);
        $statement->execute();

And it works. I feel like I'm missing something but can't figure it out. So clearly there's a problem with binding I tried different variations such as writing one of the variable manually, but no luck so far.

Thanks in advance for any help...

Answer 1

You can't use parameters as table names in PDO, so you will have to change this to avoid that. This is not a limitation of PDO, but a limitation of MySQL directly. The manual states that

Parameter markers can be used only where data values should appear, not for SQL keywords, identifiers, and so forth.

Table and column names are identifiers, so using placeholders for them is not supported. See this question for an alternative method.