title: How to check string on it has mixed letters in one word?
body: <p>How to check string on it has mixed (latin &amp; cyrillic) symbols in one word?
  For example:</p>
  
  <pre><code>$str = 'This is test string'; //ok
  $str = 'This is тест string'; //ok
  $str = 'This is тестstring'; // &lt;-- fail, how to detect this?
  </code></pre>
  
  <p>More examples:</p>
  
  <pre><code>$str = 'This is тест_123 string'; //ok
  $str = 'This is {тест}_string'; //fail
  $str = 'Абвгabcd'; //fail
  $str = 'Абвг_abcd'; //fail
  $str = 'Абвг abcd'; //ok
  $str = 'This sentence has русское word'; //ok
  $str = 'This has splittedкириллицаletters word'; //fail
  </code></pre>