How can I save a $ _session variable in an array?

I would like to ask how I could save the variable session in an array, I was trying not to add the variable to an array

<?php
session_start();
$array1 = file("ids.txt");
$test = array_push($array1, $_SESSION['u_id']);
file_put_contents("ids.txt", implode( PHP_EOL, $test));