filter.php

<?php
$contactos = array(

    array("codigo" => 1, "nombre" => "Juan Pérez",
    "telefono" => "966112233", "email" => "juanp@gmail.com"),

    array("codigo" => 2, "nombre" => "Ana López",
    "telefono" => "965667788", "email" => "anita@hotmail.com"),

    array("codigo" => 3, "nombre" => "Mario Montero",
    "telefono" => "965929190", "email" => "mario.mont@gmail.com"),

    array("codigo" => 4, "nombre" => "Laura Martínez",
    "telefono" => "611223344", "email" => "lm2000@gmail.com"),

    array("codigo" => 5, "nombre" => "Nora Jover",
    "telefono" => "638765432", "email" => "norajover@hotmail.com"),
    );

$filtrados = [];

$filtrarPor = strtolower($_GET["dominio"] ?? "");

    foreach($contactos as $contacto){
        if(strpos(strtolower($contacto["email"]), $filtrarPor) !== FALSE){
            $filtrados[] = $contacto;
        }
    }

    print_r($filtrados);
?>