index.php

<?php
$expresion = '/page=[0-9]+/';

$pages = [
    "http://localhost/ejemplo/pagina?page=1" => "http://localhost/ejemplo/pagina?page=2",
    "http://localhost/ejemplo/pagina?page=1&foo=20" => "http://localhost/ejemplo/pagina?page=2&foo=20",
    "http://regex101.com/?page=1" => "http://regex101.com/?page=2",
    "http://regex101.com/?page=1&foo=20" => "http://regex101.com/?page=2&foo=20"
];

$page = 2;

foreach ($pages as $test => $control) {
    $result = preg_replace($expresion, "page=$page", $test);

    if ($result <> $control) {
        echo "<b>" . $result . "</b> - " . $control . "</br>";
    } else {
        echo "<b>ok</b> - $test </br>";
    }
}
?>