简介：这是isset function of PHP的详细页面，介绍了和php,php, isset isset function of PHP有关的知识、技巧、经验，和一些php源码等。
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(PHP 4, PHP 5)
isset ― Determine if a variable is set and is not NULL
检查参数的值是否已经设置并且不为NULL。
Description
bool isset ( mixed $var [, mixed $... ] )
Determine if a variable is set and is not NULL .
If a variable has been unset with unset() , it will no longer be set. isset() will return FALSE if testing a variable that has been set to NULL . Also note that a NULL byte ("\0" ) is not equivalent to the PHP NULL constant.
检查参数的值是否已经设置并且不为NULL。
如果参数调用unset()而不被设置，那它以后也不会被设置。如果参数设置为NULL，调用isset()会返回FALSE。而且NULL字节不等同PHP中NULL的常量值。
If multiple parameters are supplied then isset() will return TRUE only if all of the parameters are set. Evaluation goes from left to right and stops as soon as an unset variable is encountered.
如果想多个参数调用isset()返回TRUE，只有这些函数都被设置过。函数会从左向右检查所有的参数，一直遇到没有设置的或者参数集末尾。
Parameters
var The variable to be checked. 被检查的参数。
... Another variable ... 另外的参数。
Return Values
Returns TRUE if var exists and has value other than NULL , FALSE otherwise.
假如参数存在或者不为空返回TRUE，其他返回FALSE。