preg_replace轮换PHP代码

preg_replace替换PHP代码
有一段PHP代码，我想让至之间的字符全部变为大写

PHP code

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a = '<a><?php $str = \'0\';
if($str==\'0\'){
echo $str;
}
?></a>';

echo preg_replace("/<a>(.*?)/es",'"</a><a>".strtoupper("\\1")."</a>"',$a);

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PHP code

$a = '<a><?php $str = \'0\';
if($str==\'0\'){
echo $str;
}
?></a>';
echo preg_replace_callback("/<a>(.*?)/s", 'foo', $a);
function foo($r) {
  return "</a><a>".strtoupper($r[1])."</a>";
} <div class="clear">
                 
              
              
        
            </div>

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