Dynamically replace the “src” attributes of all

Answer 1

If you care to go the way of DOMDocument():

$dom=new DOMDocument();
$dom->loadHTML($your_html);
$imgs = $dom->getElementsByTagName("img");
foreach($imgs as $img){
    $alt = $img->getAttribute('alt');
    if ($alt == 'pumpkin'){
        $src = 'http://myhost.com/cache/img001.gif';    
    } else if ($alt== '*'){
        $src = 'http://myhost.com/cache/img002.gif';
    } else if ($alt== 'cool image'){
        $src = 'http://myhost.com/cache/img003.jpg';
    }
    $img->setAttribute( 'src' , $src );
}