0是数字，是empty，是false，不是null，值相当于空字符串，但类型不是字符串，去空格或强制转换为字符串型时不等于空字符串
""的值相当于0，是empty，是空字符串，是false，不是null，不是数字字符串
null的值相当于空字符串，值相当于0，是empty，是false，不是字符串，不是数字0
"=="只要值相等就满足条件； "==="需要两个变量的值和类型都相等；
strval();将变量转换为字符串类型；
intval();将变量转换为整型；
is_numeric();检测变量是否为数字或数字字符串，返回true或false；
// 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) != '';";
}
//0=='',trim(0)!='',strval(0)!='' 不是空字符串 if($a === '')
{
echo "0 === '';";
}
else
{
echo "0 !=== '';";
}
//0!==='' if(empty($a))
{
echo "0 is empty;";
}
else
{
echo "0 is not empty;";
}
//0 is empty if(is_null($a))
{
echo "0 is null;";
}
else
{
echo "0 is not null;";
}
//0 is not null if(is_numeric($a))
{
echo "0 is numeric;";
}
else
{
echo "0 is not numeric;";
}
//0 is not numeric if(is_string($a))
{
echo "0 is string;";
}
else
{
echo "0 is not string;";
}
//0 is not string if(!$a)
{
echo "0 is false;";
}
else
{
echo "0 is not false;";
}
//0 is 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;";
}