PHP：mysql数据库的条件

I'm pretty new in PHP coding, and I cannot find the error in the following code... could you help me please ?

Fatal error: Cannot use object of type PDOStatement as array in C:\wamp\www\membre\inscription_post.php on line 14 Call Stack # Time Memory Function Location 1 0.0006 682688 {main}( ) ..\inscription_post.php:0

<?php
try
{
$bdd = new PDO('mysql:host=localhost;dbname=test', 'root', '');
}
catch(Exception $e)
{
die('Erreur : '.$e->getMessage());
}
$mem = $bdd -> query('SELECT * FROM membres');

while ($data = $mem -> fetch())
{
    if($mem['pseudo'] == $_POST['pseudo'])
    {
        echo "Pseudo existant";
    }
    else
    {
        $pass_hache = sha1($_POST['pass']);
        $req = $bdd->prepare('INSERT INTO membres (pseudo, pass, email, date_inscription) VALUES(?, ?, ?, CURDATE())');
        $req->execute(array($_POST['pseudo'], $pass_hache, $_POST['email']));
        header('Location: inscription.php');
        echo "Membre ajouté";
    }
}
$mem -> closeCursor();

?>

membres table structure is the following

id(=INT, primary key), 
pseudo (VARCHAR(255)), 
pass (VARCHAR(255)), 
date_inscription (date)

Thank you for your help

something like this

<?php
$dsn = 'mysql:host=localhost;dbname=test;charset=utf8';
$opt = array(
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
);
$bdd = new PDO($dsn,'root','', $opt);

$stm = $bdd->prepare('SELECT 1 FROM membres WHERE pseudo=?');
$stm->execute(array($_POST['pseudo']));
$row = $stm->fetch();
if ($row) {
    echo "Pseudo existant";
} else {
    $pass_hache = sha1($_POST['pass']);
    $sql = 'INSERT INTO membres VALUES(NULL, ?, ?, ?, CURDATE())';
    $req = $bdd->prepare($sql);
    $req->execute(array($_POST['pseudo'], $pass_hache, $_POST['email']));
    header('Location: inscription.php');
}

insert query can be wrong, depends on the table schema

if($mem['pseudo'] == $_POST['pseudo']) should be if($data['pseudo'] == $_POST['pseudo']) as you are reading each line from $mem as $data. However, this doesn't seem like the best option for doing what it looks like you are trying to do - why not search for $_POST['pseudo'] in the database directly instead of looping through them all?

if($mem['pseudo'] == $_POST['pseudo'])
    ^^^--- should be $data instead

But this is bad code. You're basically buying up the entire contents of a grocery store (your members table), driving it all home, then throwing away everything EXCEPT the one chocolate bar you wanted. You should be doing this in the DB, essentially

SELECT * FROM members WHERE pseudo=...