Detailed explanation of examples of comparison operations and logical operations in PHP

//0开头的数字字符串转数字时不会按八进制转换，而是简单地丢弃把 '0' 丢弃按数字进行比较，

  123=='0123' //true
  "123"<"0124" //true，0开头的数字字符串直接按十进制数字比较而非八进制

  "012" == 10 // false
== 10 // true
  0x12 == 18 // true
  "0x12" == 18 // true
 

 false < true; //true
 2>true; // false
 2==true; // true 
 null==0; //true
-1<0;//true
-1<null;//false ,-1 转 bool 是true

class Test{
 private $k=1;
 public function __get($propertyName){
  return 123;
 }
}

$obj = new Test();
echo json_encode(empty($obj->k)); //true
echo json_encode(isset($obj->k)); //false
echo json_encode((bool)($obj->k)); //true

echo 'abc'*1 ; //0 
echo '012'*1; //12 乘法：可以转换十六进制数，不是数字开头则返回 0
echo '0x12.123'*1; //18

echo (float)'0x12' ;//0 
echo (int)'0x12' ; //0 不能处理十六进制数
echo (float)'12abc'; //12 截取左侧字符串
echo (float)'abc'; // 0 不是数字返回0

is_numeric('0x123'); //true 能识别十六进制数
is_numeric('0x123.123'); //false 识别目标是整个字符串而非截取前面一部分

//几个转字符串的值(string)0 ; // "0"
(string)true; // "1"
(string)false; // ""
(string)null; // ""
(string)array(); // "Array"

class A {
 private $A = 'private property, $A of class A'; // This will become '\0A\0A'
 protected $C = 'protected property, $C of class A';
}

class B extends A {
 private $A = 'private property, $A of class B'; // This will become '\0B\0A'
 public $AA = 'public property, $AA of class B'; // This will become 'AA'
 protected $B = 'protected property, $B of class B';
}

$arr = (array) new B();

foreach ($arr as $key => $value) {
 echo &#39;<br />&#39;;
 echo $key .&#39;,length: &#39;.strlen($key).&#39; value: &#39;.$value;
}

BA,length: 4 value: private property, $A of class B
AA,length: 2 value: public property, $AA of class B
*B,length: 4 value: protected property, $B of class B
AA,length: 4 value: private property, $A of class A
*C,length: 4 value: protected property, $C of class A

$a = 1;
$b=0;
$b and $a = 100;
echo $a; //1
$b || $a = 200;
echo $a; //200

$str = 'ab0d';
empty($str[0]); //false
empty($str[0.5]); //false 索引被向下取整 为 0
empty($str["0.5"]); //false 索引被向下取整 为 0，PHP5.4之后不取证，判定为 true 
empty($str[2]); //true ，取得的字符为 "0"
empty($str["3"]); //false ，取得的字符为 "d"
empty($str[4]); //true ,索引超出范围,notice 警告，但 empty() 会忽略警告
empty($str['a']); // false ,左侧不包含数字字符串索引 PHP5.4之前被处理为 $str[0]，PHP5.4之后，直接为判定 true

// 数组是用标准比较运算符这样比较的
function standard_array_compare($op1, $op2)
{
 if (count($op1) < count($op2)) {
  return -1; // $op1 < $op2
 } elseif (count($op1) > count($op2)) {
  return 1; // $op1 > $op2
 }
 foreach ($op1 as $key => $val) {
  if (!array_key_exists($key, $op2)) {
   return null; // uncomparable
  } elseif ($val < $op2[$key]) {
   return -1;
  } elseif ($val > $op2[$key]) {
   return 1;
  }
 }
 return 0; // $op1 == $op2
}

$arg = 'T'; 
 $vehicle = ( ( $arg == 'B' ) ? 'bus' : 
     ( $arg == 'A' ) ? 'airplane' : 
     ( $arg == 'T' ) ? 'train' : 
     ( $arg == 'C' ) ? 'car' : 
     ( $arg == 'H' ) ? 'horse' : 
     'feet' ); 
 echo $vehicle; //horse

( $arg == 'B' ) ? 'bus' : ( $arg == 'A' ) 
         ? 'airplane' : ( $arg == 'T' ) 
                ? 'train' : ( $arg == 'C' )
                    ? 'car' : ( $arg == 'H' )
                         ? 'horse' : 'feet' ;