$uri    = "lala?marcelo=viadinho&marcelo_gay=frufru";
$param  = "utm_medium";
$concat = "[lala_e_lele]"

$arr = split('/[\?&]/', $uri);

foreach ($arr as $key) {
    $uri_out .= $key . ( preg_match('/$param/', $key ) ) ? $concat : '';
}