1 if ( ! defined('ENVIRONMENT') OR ! file_exists($file_path = BASEPATH.'config/'.ENVIRONMENT.'/database.php')

 3 $a1 = true;
 4 $a2 = false;
 5 
 6 $b1 = true;
 7 $b2 = false;
 8 
 9 var_dump($a1 AND $a2);    //false
10 var_dump($a1 && $a2);    //false
11 
12 var_dump($a1 OR $a2);    //true
13 var_dump($a1 || $a2);    //true

1  $a = $a1 OR $a2;　　　　//true　　 
2  $a = $a2 OR $a1;　　　　//false 坑1
3  $a = $a1 || $a2;　　　　  //true 
4 
5  $b = $b1 AND $b2;         //true          
6  $b = $b2 AND $b1;         //false  坑2
7  $b = $b1 && $b2;           //false

1 if ( ! defined('ENVIRONMENT') OR ! file_exists($file_path = BASEPATH.'config/'.ENVIRONMENT.'/database.php')

1 短路运用 
2 defined('ENV') or die('env not defined!');
3 defined('ENV') or define('ENV','DEVELOP');

1 举例:
2 $a = 0;
3 $b = 0;
4 if ($a =4 || $b =5){
5 　　echo $a,$b;
6 }