你可以使用类似这样的东西来转换你的字符串：

<?php

$str = '<p>Testing if urls work <a href=\"google.com\" rel=\"noopener noreferrer\" target=\"_blank\">google.com</a>google.com</p><p>Testing if urls work <a href=\"http://google.com\" rel=\"noopener noreferrer\" target=\"_blank\">google.com</a>google.com</p> this is some more content that might be added to the url <a href="https://www.google.com">www.Google.com</a> with some more text that follows.';

function checklinks($str){
    
    $str = str_replace("\\", "", $str);
    
    $callback = function($match){
        
        if (!preg_match('/https:\/\//i', $match[1])) {
            
            if (preg_match('/http:\/\//i', $match[1])) $urlstr = '<a href="https://' . substr($match[1], 7) . '"';
            elseif (preg_match('/http\/\//i', $match[1])) $urlstr = '<a href="https://' . substr($match[1], 6) . '"';
            else $urlstr = '<a href="https://' . $match[1] . '"';
        
        } else $urlstr = '<a href="' . $match[1] . '"';
        
        return $urlstr;
    };
    
    if (preg_match('/<a\s+href="([^"]+)"/i', $str)) {
        $str = preg_replace_callback('/<a\s+href="([^"]+)"/i', $callback, $str);
    }
    
    return $str;    
}

echo htmlentities($str) . '<br><br>';

echo htmlentities(checklinks($str));

?>

使用该函数后，结果将是：

<p>Testing if urls work <a href="https://google.com" rel="noopener noreferrer" target="_blank">google.com</a>google.com</p><p>Testing if urls work <a href="https://google.com" rel="noopener noreferrer" target="_blank">google.com</a>google.com</p> this is some more content that might be added to the url <a href="https://www.google.com">www.Google.com</a> with some more text that follows.