title: Make bold specific part of string
body: <p>I have an array like</p>
  
  <pre><code> $array[]="This is a test";
   $array[]="This is a TEST";
   $array[]="TeSt this";
  </code></pre>
  
  <p>I need to make the string 'test' as <code>bold</code> like</p>
  
  <pre><code> $array[]="This is a &lt;b&gt;test&lt;/b&gt;";
   $array[]="This is a &lt;b&gt;TEST&lt;/b&gt;";
   $array[]="&lt;b&gt;TeSt&lt;/b&gt; this";
  </code></pre>
  
  <p>I have tried with <code>str_replace()</code> but it is case sensitive,</p>
  
  <p><strong>Note:</strong>
  I need to make the given string bold and keep as it is.</p>