too long

This IS NOT a question but a REMARK I should think very useful.

The following piece of code

$sql="UPDATE    Adherents
         SET    Prenom='$_POST[tfPrenom]',  
                Civilite='$_POST[mdCivil]',
                .....
       WHERE No_Adherent=$LeNumero";

[followed by the usual mysqli_query() call]

would constantly fail and was extremely difficult to debug: under 'die' condition

if (!$resultat) { die ('Unable to update: error code is ' . mysqli_connect_errno() . ' described as '. >mysqli_connect_error()); }

it would return 'Unable to update: error code is 0 described as', i.e. NO ERROR CODE, NO ERROR MESSAGE.

As a matter of fact, my mistake laid in a wrongly-spelled row name (one cannot notice it here, of course — a missing trailing letter).

CONCLUSION: mysqli_query() can generate an error without appropriate error code/message.

I hope this can help some.

You are using the wrong functions.

As stated in the manual entry for mysqli_connect_errno():

Returns the last error code number from the last call to mysqli_connect().

You want mysqli_errno() and mysqli_error():

if (!$resultat) die (
  'Unable to update: error code is '
  . mysqli_errno($link) . ' described as ' . mysqli_error($link)
);

Use the sprintf instead of using variable directly into the query string.

$sql =sprintf("UPDATE Adherents SET Prenom= %s, Civilite=%s WHERE No_Adherent=%s",$_POST[tfPrenom], $_POST[mdCivil], $LeNumero);

PHP will replace the %s with the variables value.

Please try the following for detecting error:

<?php
$link = mysqli_connect("localhost", "my_user", "my_password", "world");

    /* check connection */
    if (mysqli_connect_errno()) {
        printf("Connect failed: %s
", mysqli_connect_error());
        exit();
    }

    if (!mysqli_query($link, "update ...SET a=1")) {
        printf("Errormessage: %s
", mysqli_error($link));
    }

    /* close connection */
    mysqli_close($link);
    ?>