php 0、null、empty和false之间的关系

// 判断 0 与 ''、null、empty、false 之间的关系 
$a = 0; 
　　echo "0 与 ''、 empty、null、false 之间的关系："; 
if($a == ''){ 
    echo "0 == '';"; 
}else{ 
    echo "0 != '';"; 
} 
if(trim($a) == ''){ 
    echo "trim(0) == '';"; 
}else{ 
    echo "trim(0) != '';"; 
} 
if(strval($a) == ''){ 
    echo "strval(0) == '';"; 
}else{ 
    echo "strval(0) != '';"; 
} 
if($a === ''){ 
    echo "0 === '';"; 
}else{ 
    echo "0 !=== '';"; 
} 
if(empty($a)){ 
    echo "0 is empty;"; 
}else{ 
    echo "0 is not empty;"; 
} 
if(is_null($a)){ 
    echo "0 is null;"; 
}else{ 
    echo "0 is not null;"; 
} 
if(is_numeric($a)){ 
    echo "0 is numeric;"; 
}else{ 
    echo "0 is not numeric;"; 
} 
if(is_string($a)){ 
    echo "0 is string;"; 
}else{ 
    echo "0 is not string;"; 
} 
if(!$a){ 
    echo "0 is false;"; 
}else{ 
    echo "0 is not false;"; 
} 
// 判断 '' 和 0、null、empty、false 之间的关系 
$a = ''; 
    echo "'' 和 0、empty、null、false 之间的关系："; 
if($a == 0){ 
    echo "'' == 0;"; 
}else{ 
　　echo "'' != 0;"; 
} 
if(intval($a) == 0){ 
    echo "intval('') == 0;"; 
}else{ 
    echo "intval('') != 0;"; 
} 
if(empty($a)){ 
    echo "'' is empty;"; 
}else{ 
    echo "'' is not empty;"; 
} 
if(is_null($a)){ 
    echo "'' is null;"; 
}else{ 
    echo "'' is not null;"; 
} 
if(is_numeric($a)){ 
　　echo "'' is numeric;"; 
}else{ 
　　echo "'' is not numeric;"; 
} 
if(is_string($a)){ 
    echo "'' is string;"; 
}else{ 
    echo "'' is not string;"; 
} 
if(!$a){ 
    echo "'' is false;"; 
}else{ 
    echo "'' is not false;"; 
} 
// 判断 null 和 ''、0、empty、false 之间的关系 
$a = null; 
    echo "null 和 ''、0、empty、false 之间的关系："; 
if($a == ''){ 
    echo "null == '';"; 
}else{ 
    echo "null != '';"; 
} 
if($a == 0){ 
    echo "null == 0;"; 
}else{ 
    echo "null != 0;"; 
} 
if($a === ''){ 
    echo "null === '';"; 
}else{ 
    echo "null !=== '';"; 
} 
if($a === 0){ 
    echo "null === 0;"; 
}else{ 
    echo "null !=== 0;"; 
} 
if(strval($a) == ''){ 
    echo "strval(null) == '';"; 
}else{ 
    echo "strval(null) != '';"; 
} 
if(intval($a) == 0){ 
    echo "intval(null) == 0;"; 
}else{ 
    echo "intval(null) != 0;"; 
} 
if(empty($a)){ 
    echo "null is empty;"; 
}else{ 
    echo "null is not empty;"; 
} 
if(is_numeric($a)){ 
    echo "null is numeric;"; 
}else{ 
    echo "null is not numeric;"; 
} 
if(is_string($a)){ 
    echo "null is string;"; 
}else{ 
    echo "null is not string;"; 
} 
if(!$a){ 
    echo "null is false;"; 
}else{ 
    echo "null is not false;"; 
} 
echo ""; 

运行结果：

0 与 '' empty null false 之间的关系：

 0 == '';

trim(0) != '';

strval(0) != '';

0 !=== '';

0 is empty;

0 is not null;

0 is numeric;

0 is not string;

0 is false;

'' 和 0 empty null false 之间的关系：

'' == 0;

intval('') == 0;

'' is empty;

'' is not null;

'' is not numeric;

'' is string;

'' is false;

null 和 '' 0 empty false 之间的关系：

null == '';

null == 0;

null !=== '';

null !=== 0;

strval(null) == '';

intval(null) == 0;

null is empty;

null is not numeric;

null is not string;

null is false;

需要注意的是0，null，false都是empty，