how split a mkv with subtitles

You can use mkvtoolnix, then go to -> output, and -> splitting -> by parts based on timestamps -> and there you can write the beginning and the end of the poart you want extract (without spaces: like 1:02:07-1:04:40, for example).

show “alt” attribute after an image

You can use a javascript, like this

<script type="text/javascript">
// img[style]').each(function(){
       $el = $(this);
       var style = $el.attr('style');
       $el.attr('style','');
       $el.parent().attr('style',style);
    }); //Moves the inline styles

      $("img").each(function(){
          var title = this.alt;
          $(this).after(''+ title +'');
      }); //Adds the dynamic captions.
 });
 //]]>
</script>

mysql convert a column to utf-8

You can use this mysql query code:

ALTER TABLE yourtable MODIFY
    yourcol VARCHAR(255)
      CHARACTER SET utf8
      COLLATE utf8_general_ci;

There are more problems with ENUM and SET type fields.

For ENUM you have to specify all the field values, no matter how many they are

ALTER TABLE yourtable MODIFY
    yourcol ENUM('val1','val2',etc)
      CHARACTER SET utf8
      COLLATE utf8_general_ci;

For SET the problems rise from the name SET, so you have to do this command

ALTER TABLE yourtable MODIFY
    yourcol CHAR(255)
      CHARACTER SET utf8
      COLLATE utf8_general_ci;

You will be able afterwards to modify CHAR to SET (with your previous values) without losing your data.