PHP/MySQL : why my query doesn't work?

Solution:

mysqli prepared statements should work. Just change a couple of lines,

$reqVerif = $bdd->prepare('SELECT ID_utilisateur FROM utilisateurs WHERE Adresse_email = ?');
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You are only selecting ONE string here ,so only list it once:

$reqVerif->bind_param("s",$email);
$reqVerif->execute();
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For your select it is easiest to bind your results, like so:

$reqVerif->bind_result($ID_utilisateur);
$reqVerif->fetch();
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Then you can echo out your result like so:

echo $ID_utilisateur;
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and then close your object:

$reqVerif->close();
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(of course you can do it like you did above; I just simplified this into its elements)

Edit: if you want to have it check if there are any rows (which is what it appears), you will need to store the result after the execution with this row:

$reqVerif->store_result();
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and then you can do something like this:

$num=$reqVerif->num_rows;
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and then check it:

if ($num > 0) {

    echo $ID_utilisateur;
}
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and then you can free the result before you close the object with this:

$reqVerif->free_result;
$reqVerif->close();
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