- print_r: documentation ( source)
- array_merge: documentation ( source)
<?php
$a = [
'gateway' => 'a',
'email' => 'b',
'firstname' => 'c',
'lastname' => 'd',
'gender' => 'e',
'birthDate' => 'f',
'city' => 'g',
'zip' => 'h',
'client' => 'i',
];
$b = [
'gateway' => '1',
'email' => '2',
'firstname' => '',
'lastname' => '3',
];
print_r( array_merge($a, $b) );