The isset() function checks whether a variable is set, which means that it has to be declared and is not NULL. This function returns true if the variable exists and is not NULL, otherwise it returns false.

isset() will return false when checking a variable that has been assigned to null. Also note that a null character ( "\0" ) is not equivalent to the PHP null constant. If multiple parameters are supplied then isset() will return true only if all of the parameters are considered set.

isset() 函数用于检测变量是否已设置并且非 NULL。 如果已经使用 unset() 释放了一个变量之后，再通过 isset() 判断将返回 FALSE。 若使用 isset() 测试一个被设置成 NULL 的变量，将返回 FALSE。

2024年9月24日 · The isset() function in PHP checks whether a variable is declared and not NULL. It returns true if the variable exists and has a non-NULL value, and false otherwise, without modifying the variable. Syntax bool isset( mixed $var [, mixed $... ] ) Parameters: This function accept one or more parameter as mentioned above and described below:

ISSET returns TRUE if the variable exists and has a value other than NULL. That means variables assigned a "", 0, "0", or FALSE are set, and therefore are TRUE for ISSET . EMPTY checks to see if a variable is empty.

So use isset() when you're concerned about the existence of a variable. And use empty when you're testing for true or false. If the actual type of emptiness matters, use is_null and ===0 , ===false , ==='' .

isset() returns true if a string index is valid or false otherwise. isset() returns true if all variables are set and not null. It’ll stop evaluating once it encounters an unset variable.