isset function of PHP
isset(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
boolisset( mixed$var[, mixed$...] )
Determine if a variable is set and is not NULL.
If a variable has been unset with unset(), it will nolonger be set. isset()will return FALSEif testing avariable that has been set to NULL. Also note that a NULLbyte("\0") is not equivalent to the PHP NULLconstant.
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()willreturn TRUEonly if all of the parameters are set. Evaluation goes fromleft 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 TRUEif varexists and has value otherthan NULL, FALSEotherwise.
假如参数存在或者不为空返回TRUE，其他返回FALSE。
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