eoigal

<?php

$associative_array = ['firstname' => 'John', 'lastname' => 'Smith', 'DOB' => '2000-10-10', 'country' => 'Ireland' ];

$subset = array_intersect_key( $associative_array, array_flip( [ 'lastname', 'country' ] ) );

print_r( $subset );

// Outputs...
// Array ( [lastname] => Smith [country] => Ireland );