您应该尝试以下操作：

function replace_tag($string){
    $search = array(\'<p>\', \'</p>\');
    $replace = array(\'<h2>\', \'</h2>\');
    echo str_replace($search, $replace, $string);
    return $string;
}
add_filter(\'the_excerpt\', \'replace_tag\');

function replace_tag($string){
    $replace = array(
        \'<p>\' => \'<h2>\',
        \'</p>\' => \'</h2>\'
    );
    $string = str_replace(array_keys($replace), $replace, $string);
    return $string;
}
add_filter(\'the_excerpt\', \'replace_tag\');