How to store values from foreach loop into an array?

Use

$items[] = $username;

Try

$items = array_values ( $group_membership );

<?php 
$items = array();
$count = 0;
foreach($group_membership as $i => $username) { 
 $items[$count++] = $username; 
} 
print_r($items); 
?>

This is how you do it:

foreach($orders as $item){
   $transactionIds[]  =   $item->generated_order_id;
}

dump($transactionIds);

This is assuming $orders collection has a field called 'generated_order_id'.

You can try to do my answer,

you wrote this:

<?php
foreach($group_membership as $i => $username) {
    $items = array($username);
}

print_r($items);
?>

And in your case I would do this:

<?php
$items = array();
foreach ($group_membership as $username) { // If you need the pointer (but I don't think) you have to add '$i => ' before $username
    $items[] = $username;
} ?>

As you show in your question it seems that you need an array of usernames that are in a particular group :) In this case I prefer a good sql query with a simple while loop ;)

<?php
$query = "SELECT `username` FROM group_membership AS gm LEFT JOIN users AS u ON gm.`idUser` = u.`idUser`";
$result = mysql_query($query);
while ($record = mysql_fetch_array($result)) { \
    $items[] = $username; 
} 
?>

while is faster, but the last example is only a result of an observation. :)

$items=array(); 
$j=0; 

foreach($group_membership as $i => $username){ 
    $items[$j++]=$username; 
}

Just try the above in your code .

Just to save you too much typos:

foreach($group_membership as $username){
        $username->items = array(additional array to add);
    }
    print_r($group_membership);

this question seem quite old but incase you come pass it, you can use the PHP inbuilt function array_push() to push data in an array using the example below.

<?php
    $item = array();
    foreach($group_membership as $i => $username) {
        array_push($item, $username);
    }
    print_r($items);
?>