如何使用WordPress中的过滤器将类添加到html元素？

Question

I am changing the current theme of my blog and I wish to add a class to all my input fields.

I am not well versed with regex codes, so I end up in a lot of trouble whenever I need to make things like these happen.

type="submit"

I want to add class btn needs to be added to <input type="submit">

Old Code

<input type="submit" id="some_id" name="something" class="some_class" value="some_value" />

New Code

<input type="submit" id="some_id" name="something" class="btn some_class" value="some_value" />

---

and

Similarly, I want the class input to be added to <input type="text"> and <input type="textarea">

Old Code

<input type="text" id="some_id" name="something" class="some_class" value="some_value" />
<textarea id="some_id" name="something" class="some_class" value="some_value" />

New Code

<input type="text" id="some_id" name="something" class="input some_class" value="some_value" />
<textarea id="some_id" name="something" class="input some_class" value="some_value" />

Answer 1

Given your constraints, the cleanest way you can do what you want with PHP and stay within the Wordpress framework is to use DOMDocument. While it's POSSIBLE to rely on regular expressions, it's very sloppy and you can run into more problems than what you started with.

Place this in your functions.php file, and it should do everything you need:

add_filter('the_content', 'add_text_input_classes', 20);
function add_text_input_classes($content)
{
    $doc = new DOMDocument(); //Instantiate DOMDocument
    $doc->loadHTML($content); //Load the Post/Page Content as HTML
    $textareas = $doc->getElementsByTagName('textarea'); //Find all Textareas
    $inputs = $doc->getElementsByTagName('input'); //Find all Inputs
    foreach($textareas as $textarea)
    {
        append_attr_to_element($textarea, 'class', 'input');
    }
    foreach($inputs as $input)
    {
        $setClass = false;
        if($input->getAttribute('type') === 'submit') //Is the input of type submit?
            $setClass = 'btn';
        else if($input->getAttribute('type') === 'text') //Is the input of type text?
            $setClass = 'input';

        if($setClass)
            append_attr_to_element($input, 'class', $setClass);
    }
    return $doc->saveHTML(); //Return modified content as string
}
function append_attr_to_element(&$element, $attr, $value)
{
    if($element->hasAttribute($attr)) //If the element has the specified attribute
    {
        $attrs = explode(' ', $element->getAttribute($attr)); //Explode existing values
        if(!in_array($value, $attrs))
            $attrs[] = $value; //Append the new value
        $attrs = array_map('trim', array_filter($attrs)); //Clean existing values
        $element->setAttribute($attr, implode(' ', $attrs)); //Set cleaned attribute
    }
    else
        $element->setAttribute($attr, $value); //Set attribute
}