what is eval function in php code example

Example: php eval

<?php
  $string = "beautiful";
  $time = "winter";
  $str = 'This is a $string $time morning!';
  echo $str. "<br>";
  eval("\$str = \"$str\";");
  echo $str;
?>
// This is a $string $time morning!
// This is a beautiful winter morning!