<?PHP
function addClass($htmlString = '',$newClass) {
    $pattern = '/class="([^"]*)"/';

    // class attribute set
    if (preg_match($pattern,$htmlString,$matches)) {
        $definedClasses = explode(' ',$matches[1]);
        if (!in_array($newClass,$definedClasses)) {
            $definedClasses[] = $newClass;
            $htmlString = str_replace($matches[0],sprintf('class="%s"',implode(' ',$definedClasses)),$htmlString);
        }
    }

    // class attribute not set
    else {
        $htmlString = preg_replace('/(\<.+\s)/',sprintf('$1class="%s" ',$newClass),$htmlString);
    }

    return $htmlString;
}

echo addClass('<a class="abc">','newClass');
echo addClass('<p style=display:block>','newClass');

使用http://php.net/manual/en/book.dom.php示例

<?PHP
function addClass($node = null,$className) {
    $result = false;

    if (is_string($node)) {
        $dom = DOMDocument::loadXml($node);
        if ($dom instanceof DOMDocument) {
            $definedClasses = explode(' ',$dom->documentElement->getAttribute('class'));
            if (!in_array($className,$definedClasses)) {
                $dom->documentElement->setAttribute(
                    'class',$dom->documentElement->getAttribute('class') . ' ' . $className
                );
            }

            $result = $dom->saveXml($dom->documentElement,true);
        }
    }
    elseif ($node instanceof DOMElement) {
        // this code repetition,could of course be avoided using some more sophisticated structures 
        $definedClasses = explode(' ',$node->getAttribute('class'));
        if (!in_array($className,$definedClasses)) {
            $node->setAttribute('class',$node->getAttribute('class') . ' ' . $className);
        }

        $result = $node;
    }

    return $result;
}

// using a string as input
echo addClass('<a class="abc"></a>','newClass');

// using a DOMElement as input
$dom = DOMDocument::loadHtml('<div><a id="something"></a></div>');
$xpath = new DOMXPath($dom);

$node = $xpath->query('//*[@id="something"]')->item(0);
if ($node instanceof DOMElement) {
    addClass($node,'newClass');
    echo $dom->saveXml($node,true);
}

我故意不使用loadHTML(在函数内部)来防止不得不深入到自动生成的html结构中来找到实际给定的$htmlString.这当然意味着$htmlString必须格式正确.