echo 'fullstackpm'; // 正常输出：fullstackpm
echo 'fullstackpm', ' is ', 'Good!'; // 正常输出：fullstackpm is Good!
echo ('fullstackpm'); // 正常输出：fullstackpm
echo ('fullstackpm', ' is ', 'Good!'); // 报错了

print 'fullstackpm'; // 正常输出：fullstackpm
print 'fullstackpm', ' is ', 'Good!'; // 错误
$r = print ('fullstackpm'); // 正常输出：fullstackpm
print $r; // 输出1

$str = 'My name is %s, I\'m %d years old.';
printf($str, 'fullstackpm', 1002); // 直接输出：My name is fullstackpm, I'm 1002 years old.

$s = sprintf($str, 'WoW', 12); // 这里不会输出
print $s; // 输出：My name is WoW, I'm 12 years old.

$str = [
  "a",
  1 => "b",
  "3" => "c",
  "show"=>'d'
];

print_r($str)
// 输出
/**
  Array
  (
    [0] => a
    [1] => b
    [3] => c
    [show] => d
  )
*/

$s = print_r($str, true); // 此处不会输出
echo $s;
// 输出
// 注意，输出流不在ob_start()中，测试本段请不要有其他任何输出
/**
  Array
  (
    [0] => a
    [1] => b
    [3] => c
    [show] => d
  )
*/

$str = [
  "a",
  1 => "b",
  "3" => "c",
  "show"=>'d'
];

var_dump($str);
// 输出
/**
  array(4) {
   [0] =>
   string(1) "a"
   [1] =>
   string(1) "b"
   [3] =>
   string(1) "c"
   'show' =>
   string(1) "d"
  }
*/

var_export($str);
// 输出
/**
  array (
   0 => 'a',
   1 => 'b',
   3 => 'c',
   'show' => 'd',
  )
*/