img-regexp.php

<?php

$html = '<div><img alt="" src="http://example.com/" alt="test"></div>';
$imgPattern = '/<img\s(.*)\/?>/U';
preg_match_all($imgPattern, $html, $matches, PREG_SET_ORDER);

$uId = 1;
foreach ($matches as $match) {
    $img = trim($match[0]);
    $attrContent = trim($match[1]);
    
    preg_match('/src=\"(.*)\"/', $attrContent, $srcMatch);
    if (!empty($srcMatch)){
        $src = $srcMatch[1];
        $imgAttr = "Image-". $uId;
        $out = '<a href="'. $src .'" data-lightbox="'. $imgAttr .'">'. $img .'</a>';
        
        $html = str_replace($img, $out, $html);
    }
    
    $uId += 1;
}

print_r($html);