preg_replace for url links

Ok i am using this code for replace url link:

$html = preg_replace('/gallery\.php\?show=([0-9]+)/i','gallery/$1',$html);

with this code show me this result: gallery?show=5

i need this result: gallery/5

Huh?

<?php

$html = 'gallery.php?show=5';

$html = preg_replace('/gallery\.php\?show=([0-9]+)/i','gallery/$1',$html);

echo $html;

// outputs gallery/5

?>

Then what's wrong with my site show me gallery?show=5

.htaccess

RewriteRule ^gallery/([0-9]+)/?$  gallery.php?show=$1 [QSA,L]

If i use this code then show: gallery?show=5

if i remove  $html = str_replace('gallery.php','gallery',$html); then show me gallery/5

How can fix it ?

$html = str_replace('gallery.php','gallery',$html);
$html = preg_replace('/gallery\.php\?show=([0-9]+)/i','gallery/$1',$html);