为什么add_filter（）被应用两次？

function replace_text_wps($text){
        $replace = array(
            // 'WORD TO REPLACE' => 'REPLACE WORD WITH THIS'
            'wordpress' => '<a href="#">wordpress</a>',
            'excerpt' => '<a href="#">excerpt</a>',
            'function' => '<a href="#">function</a>'
        );
        $text = str_replace(array_keys($replace),
    $replace, $text);
        return $text; }

    add_filter('the_content','replace_text_wps');
    add_filter('the_excerpt','replace_text_wps');

This code is used to replace some words, why is he using add_filter() function twice, is he wrong?

Also, what does the line $text = str_replace(array_keys($replace), $replace, $text) mean?

$text = str_replace(array_keys($replace), $replace, $text);

Replaces all given keys with $replace in $text string

this code is just filtering two different strings.

$text = str_replace(array_keys($replace), $replace, $text);

This line searches for all array keys from $replace and replaces them with their respective value.

It's basically a shorter/nicer way for foreach($replace as $s => $r) $text = str_replace($s, $r, $text);

add_filter('the_content','replace_text_wps');
add_filter('the_excerpt','replace_text_wps');

He's applying the filter to the content of the post as well as the excerpt (Usually separate from the post body. Separately filled in). Generally you only use one of these in a blog listing so he's covering all his bases by applying it to both.

$text = str_replace(array_keys($replace), $replace, $text);

// 'WORD TO REPLACE' => 'REPLACE WORD WITH THIS'

Then, he's just doing a string replace : http://php.net/manual/en/function.str-replace.php

Basically, if your post content has any of the following words wordpress, excerpt, excerpt it will replace that word with a link that is wrapped arounf the word.