PHP如何仅对正则匹配的特定分组执行替换操作？

我需要在纯文本中搜索关键词和关键短语，通过包裹<span>标签实现高亮效果，这部分功能已实现，但要避免高亮已经处于span标签内的内容。

我找到一个可行的JavaScript实现逻辑，对应代码示例如下：

var subject = 'Red apples are my <span class="highlight">favourite apples.</span>';
var regex = /<span.*?<\/span>|(\bapples\b)/g;
replaced = subject.replace(regex, function(m, group1) {
    if (group1 == "" ) return m;
    else return "<span class=\"highlight\">" + group1 + "</span>";
});
document.write("<br>*** Replacements ***<br>");
document.write(replaced);

但我需要用PHP实现该功能，尝试转换逻辑后写出了以下可运行代码：

$text = 'Red apples are my <span class="highlight">favourite fruit.</span> <span class="highlight">Red apples are my favourite fruit.</span> I love red apples.';
$pattern = '/<span.*?<\/span>|(\bapples\b)/i';

preg_match_all($pattern, $text, $matches);

print_r($matches[0]);
print_r($matches[1]);

代码输出结果：

Array ( [0] => apples [1] => favourite fruit. [2] => Red apples are my favourite fruit. [3] => apples ) 
Array ( [0] => apples [1] => [2] => [3] => apples )

能看出正则表达式有效，但我无法像JavaScript示例那样，仅对第二个分组执行替换操作。希望有人帮忙将上述JavaScript示例转换为可运行的PHP代码，同时让我学习更多PHP正则函数的用法。

感谢@SashaKondrashov提供的可行方案：

$text = 'Red apples are my <span class="highlight">favourite fruit.</span> <span class="highlight">Red apples are my favourite fruit.</span> I love red apples.';
$pattern = '/<span.*?<\/span>|(\bapples\b)/i';
    
$result = preg_replace_callback($pattern, function ($matches) {
    if (count($matches) == 1) return $matches[0];
    return "<span>{$matches[1]}</span>";
}, $text);
    
print_r($result);

内容的提问来源于stack exchange，提问作者kjaer108