How to store values from foreach loop into an array?

Declare the $items array outside the loop and use $items[] to add items to the array:

$items = array();
foreach($group_membership as $username) {
 $items[] = $username;
}

print_r($items);

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); 
?>

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