Select multiple rows from table using checkboxes - php

Question

I'm trying to use checkboxes for multiple sql query with SELECT.
I have 3 tables, one table for groups one for users and one to connect the user to different groups called groupreluser. Based on what groups are checked, I want it to print out the phonenumber of each user that are a member of those groups.
The checkboxes are created based on the content of the table groups
<form method="post">
<?php
$sql = "SELECT * FROM groups WHERE groupname LIKE '%minor%'";
$result = $conn->query($sql);
if($result->num_rows > 0){
while($row = $result->fetch_assoc()){
echo '<input type="checkbox" name="checked[]" value="'.$row['group_id'].'">'
. $row['groupname'] . '</br>';
}
}
?>
<input type="submit" name="submit" value="submit">
</form>
Code with the sql query to get the phonenumbers.
if(isset($_POST['submit'])){
$check = $_POST['checked'];
if(!empty($check)){
foreach($check as $sel){
$sel = mysqli_real_escape_string($conn, $sel);
$sql = "
SELECT
groups.group_id,
groups.groupname,
groupreluser.user_id,
groupreluser.group_id,
users.user_id,
users.name,
users.phone
FROM
groupreluser
JOIN
groups
ON
groups.group_id = groupreluser.group_id
JOIN
users
ON
users.user_id = groupreluser.user_id
WHERE
groups.group_id = '$sel'
";
$res = $conn->query($sql);
if($res->num_rows > 0){
while($row = $res->fetch_assoc()){
echo $row['phone'] . '</br>';
}
}
}
}
}
It works fine as long as I dont use the checkboxes but adds the group_id manually in the SELECT statement. Any idea of why this isn't working? Am I missing someting? Dont know if this is the best way to do it though...
Let me know if this is unclear, and I'll try to explain it better

Answer 1

This line probably doesn't work:
$check = mysqli_real_escape_string($conn, $_POST['checked']);
$_POST['checked'] is an array. So you have do the line above for each item in the array.

Answer 2

Your $_POST['submit'] is array so directly passing it to mysqli_real_escape_string won't work. But this approach will do the work:
if(isset($_POST['submit'])){
$check = $_POST['check'];
if(!empty($check)){
foreach($check as $sel) {
$sel = mysqli_real_escape_string($conn, $sel);
$sql = "
SELECT
groups.group_id,
groups.groupname,
groupreluser.user_id,
groupreluser.group_id,
users.user_id,
users.name,
users.phone
FROM
groupreluser
JOIN
groups
ON
groups.group_id = groupreluser.group_id
JOIN
users
ON
users.user_id = groupreluser.user_id
WHERE
groups.group_id = '$sel'
";
$res = $conn->query($sql);
...........