我的php主题标签标识符功能不起作用

function hashTagFinder($str)
{
    $hash= explode(" ", substr($str, strpos($str, "#")));

    $i=0;

    foreach ($hash as  $tag) {

        $finder=substr_count($tag, "#");

        if ($finder>=1 ) { 
            $fArray = array([$i]=> $tag);
        }               
        $i++;                           
    }
}

I am a beginner and I want to make a hashtag finder function.

I am trying to do it this way but it looks like I can't use object and arrays to define keys.

function convertHashtags($str) {
    $regex = "/#+([a-zA-Z0-9_]+)/";
    $str = preg_replace($regex, '<a href="insert taglink">$0</a>', $str);
    return($str);
}

This code return an array with all #hashtags in a string:

  $str= 'your string with a #hashtag inside'; 
  preg_match_all('/#\w+/', $str, $match);
  var_dump($match); /* print the result */