使用PHP中的循环替换文本

I am trying to loop through every [footnote] and replace it with some HTML. Here is some sample text:

Hello [footnote], how are you [footnote], what are you up to [footnote]?

And using preg_match_all to create a count:

$match_count = preg_match_all("/\[footnote]/", $content);

I then use this count as a loop, to find and replace the text with appropriate HTML:

for ($i=0; $i < $match_count; $i++) { 
   $new_content = str_replace('[footnote]', "<span class='footnote'>$i</span>", $content);
}

However, after, when I echo $new_content; each [footnote] has the same number, 2:

<span class="footnote">2</span>
<span class="footnote">2</span>
<span class="footnote">2</span>

Would anyone know why this number is not incrementing? This is what I want

<span class="footnote">1</span>
<span class="footnote">2</span>
<span class="footnote">3</span>

str_replace replaces everything at once, you need preg_replace which supports $limit (=number of replacements to make):

$content = "Hello [footnote], how are you [footnote], what are you up to [footnote]?";

$i = 0;
do {
    $i++;
    $content = preg_replace('~\[footnote\]~', "<span>$i</span>", $content, 1, $count);
} while($count);

print $content;

Note that the 5th parameter, $count makes your counting code superfluous - we just keep replacing until no more replacement could be made.

Since you are trying to replace a literal string, you can avoid to use a regex. Example:

$str = 'Hello [footnote], how are you [footnote], what are you up to [footnote]?';

$arr = explode('[footnote]', $str);
$count = 1;

$result = array_reduce($arr, function ($carry, $item) use (&$count) {
     return (isset($carry)) 
         ? $carry . '<span class="footnote">' . $count++ . '</span>' . $item
         : $item;   
});

print_r($result);

you can do like this

$i = 0;
preg_replace_callback('/[footnote]/', 'replaces_counter', $content);

function replaces_counter($matches) {
  global $i;
  return "<span class='footnote'>".$i++."</span>";
}