PDO query returning False half the time

I have a problem with PDO mysql queries

HEre is my simple code (not a real situation just to show you the problem)

$connexion = new PDO(SQL_DSN, SQL_USERNAME, SQL_PASSWORD);
$stmt = $connexion->prepare('SELECT * from missions,platforms WHERE missions_platforms_id = exl_platforms_id AND missions_id = :mission');
$stmt->execute(array(":mission" => '1'));
$ligne = $stmt->fetch(PDO::FETCH_ASSOC);
var_dump($stmt);

$stmt = $connexion->prepare('SELECT * from missions,platforms WHERE missions_platforms_id = exl_platforms_id AND missions_id = :mission');
var_dump($stmt);
$stmt = $connexion->prepare('SELECT * from missions,platforms WHERE missions_platforms_id = exl_platforms_id AND missions_id = :mission');
var_dump($stmt);

and the output :

object(PDOStatement)[2]
 public 'queryString' => string 'SELECT * from m开发者_开发问答issions,platforms WHERE missions_platforms_id = platforms_id AND missions_id = :mission' (length=122)

boolean false

object(PDOStatement)[2]
 public 'queryString' => string 'SELECT * from missions,platforms WHERE missions_platforms_id = platforms_id AND missions_id = :mission' (length=122)

Why the 2nd var_dump return false ?

If I do more times the same schema it is returning false half the time...

Thanks you

Try to close your result set cursor after reading the result set with PDOStatement::fetch() using PDOStatement::closeCursor() such as:

$connexion = new PDO(SQL_DSN, SQL_USERNAME, SQL_PASSWORD);
$stmt = $connexion->prepare('SELECT * from missions,platforms WHERE missions_platforms_id = exl_platforms_id AND missions_id = :mission');
$stmt->execute(array(":mission" => '1'));
$ligne = $stmt->fetch(PDO::FETCH_ASSOC);
var_dump($stmt);
$stmt->closeCursor();

$stmt = $connexion->prepare('SELECT * from missions,platforms WHERE missions_platforms_id = exl_platforms_id AND missions_id = :mission');
var_dump($stmt);
$stmt = $connexion->prepare('SELECT * from missions,platforms WHERE missions_platforms_id = exl_platforms_id AND missions_id = :mission');
var_dump($stmt);

The first statement is the only statement you actually fetch a result from.