将字符串中的每个字母都包装在一个标记中，避免使用HTML标记

您可以使用([^'s>])(?!(?:[^<>]*)?>)正则表达式来实现结果。要启用Unicode支持，只需将其与u选项一起使用即可：

<?php
   $re = "/([^''s>])(?!(?:[^<>]*)?>)/u"; 
   $str = "Hi <br> there."; 
   $subst = "<span>$1</span>"; 
   $result = preg_replace($re, $subst, $str);
   echo $result;
?>

在这里，您可以找到regex的解释和演示。

请参阅不支持Unicode的示例程序，这里有一个支持Unicode的程序（区别在于u选项）。

您可以考虑使用DOMDocument来解析HTML，并仅将字符包装在DOMText节点的值中。请参阅代码中的注释。

// Define source
$source = 'H&iuml; <br/> thérè.';
// Create DOM document and load HTML string, hinting that it is UTF-8 encoded.
// We need a root element for this so we wrap the source in a temporary <div>.
$hint = '<meta http-equiv="content-type" content="text/html; charset=utf-8">';
$dom = new DOMDocument();
$dom->loadHTML($hint . "<div>" . $source . "</div>");
// Get contents of temporary root node
$root = $dom->getElementsByTagName('div')->item(0);
// Loop through children
$next = $root->firstChild;
while ($node = $next) {
    $next = $node->nextSibling; // Save for next while iteration
    // We are only interested in text nodes (not <br/> etc)
    if ($node->nodeType == XML_TEXT_NODE) {
        // Wrap each character of the text node (e.g. "Hi ") in a <span> of
        // its own, e.g. "<span>H</span><span>i</span><span> </span>"
        foreach (preg_split('/(?<!^)(?!$)/u', $node->nodeValue) as $char) {
            $span = $dom->createElement('span', $char);
            $root->insertBefore($span, $node);
        }
        // Drop text node (e.g. "Hi ") leaving only <span> wrapped chars
        $root->removeChild($node);
    }
}
// Back to string via SimpleXMLElement (so that the output is more similar to
// the source than would be the case with $root->C14N() etc), removing temporary
// root <div> element and space-only spans as well.
$withSpans = simplexml_import_dom($root)->asXML();
$withSpans = preg_replace('#^<div>|</div>$#', '', $withSpans);
$withSpans = preg_replace('#<span> </span>#', ' ', $withSpans);
echo $withSpans, PHP_EOL;

输出：

<span>H</span><span>ï</span> <br/> <span>t</span><span>h</span><span>é</span><span>r</span><span>è</span><span>.</span>

你可以试试。。。

<?php
  $str = "Hi <br> there.";
  $newstr = "";
  $notintag = true;
  for ($i = 0; $i < strlen($str); $i++) {
    if (substr($str,$i,1) == "<") {
      $notintag = false;
    }
    if (($notintag) and (substr($str,$i,1) != " ")) {
      $newstr .= "<span>" . substr($str,$i,1) . "</span>";
    } else {
      $newstr .= substr($str,$i,1);
    }
    if (substr($str,$i,1) == ">") {
      $notintag = true;
    }

  }
  echo $newstr;
?>