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$all_posts = get_posts(
    \'posts_per_page\' => -1,
    \'post_type\' => \'post\'
);

foreach ( $all_posts as $single ) {
    wp_update_post( array(
        \'ID\' => $single->ID,
        \'post_title\' => to_title_case( $single->post_title ) // see function below
    ));
}

将字符串转换为“标题大小写”并且，虽然与WP无关，但为了完整起见：

function to_title_case( $string ) {
     /* Words that should be entirely lower-case */
     $articles_conjunctions_prepositions = array(
          \'a\',\'an\',\'the\',
          \'and\',\'but\',\'or\',\'nor\',
          \'if\',\'then\',\'else\',\'when\',
          \'at\',\'by\',\'from\',\'for\',\'in\',
          \'off\',\'on\',\'out\',\'over\',\'to\',\'into\',\'with\'
     );
     /* Words that should be entirely upper-case (need to be lower-case in this list!) */
     $acronyms_and_such = array(
         \'asap\', \'unhcr\', \'wpse\', \'wtf\'
     );
     /* split title string into array of words */
     $words = explode( \' \', mb_strtolower( $string ) );
     /* iterate over words */
     foreach ( $words as $position => $word ) {
         /* re-capitalize acronyms */
         if( in_array( $word, $acronyms_and_such ) ) {
             $words[$position] = mb_strtoupper( $word );
         /* capitalize first letter of all other words, if... */
         } elseif (
             /* ...first word of the title string... */
             0 === $position ||
             /* ...or not in above lower-case list*/
             ! in_array( $word, $articles_conjunctions_prepositions ) 
         ) {
             $words[$position] = ucwords( $word );
         }
     }         
     /* re-combine word array */
     $string = implode( \' \', $words );
     /* return title string in title case */
     return $string;
}

显然，这两个单词列表都可以扩展——小写列表尤其是更多的介词，首字母缩略词则是当前网站上经常使用的词。

无论如何，特定于WP的部分只是上面的代码块。