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<!DOCTYPE html>
<html>
<body>
<?php
function printFormatted(callable $format, $str) {
  echo $format($str);
  echo "<br>";
}
class MyClass {
  public static function ask($str) {
    return $str . "?";
  }
  public function brackets($str) {
    return "[$str]";
  }
}
// An anonymous function
$func = function($str) { return substr($str, 0, 5); };
printFormatted($func , "Hello World");
// A string containing the name of a function
printFormatted("strtoupper", "Hello World");
// An array describing a static class method
printFormatted(["MyClass", "ask"], "Hello World");
// An array describing an object method
$obj = new MyClass();
printFormatted([$obj, "brackets"], "Hello World");
?>
</body>
</html>
Hello
HELLO WORLD
Hello World?
[Hello World]