**文本中的粗体文字**标记php [复制]

Question

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I'm working in this function that colors an username in case it exists @username within an string.

PHP

$string="this **text** contains a second **content**"

I want to bold the text contained within text

I tried but nothing

 function bold_word($string) {
   return preg_replace('/(\**+)/', "<b>$1</b>", $string);
    }

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