<?
$first_array[0][] = 'User1';
$first_array[0][] = 5;
$first_array[1][] = 'User2';
$first_array[1][] = 3;
 
$secnd_array[0][] = 3;
$secnd_array[0][] = 10.00;
$secnd_array[1][] = 5;
$secnd_array[1][] = 47.00;
 
// Make the user_id the key of the array
foreach ($secnd_array as $sca) {
    $searchable_second_array[ $sca[0] ] = $sca[1];
}
// Modify the original array
array_walk($first_array, function(&$a) use ($searchable_second_array) {
    // Here we find the second element of the first array in the modified second array :p
    $a[] = $searchable_second_array[ $a[1] ];
});
 
print_r($first_array);

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