$testo = "doij:roll:oawkdk:)maodm:fagiano:";

$emoticons = Array( ":roll:", ":)", ":fagiano:" );
$images = Array( "[img]roll.gif[/img]", "[img]smile.gif[/img]", "[img]fagiano.gif[/img]" );




$testo = str_replace( $emoticons, $images, $testo );
echo $testo;