What does isset mean in php?

isset function in php

(PHP 4, PHP 5, PHP 7)

isset — Detection Whether the variable has been set and is not NULL

Explanation (recommended learning: PHP video tutorial)

isset ( mixed $var [, mixed $... ] ) : bool<br/>

Detects whether the variable is set and is not NULL.

If a variable has been released using unset(), it will no longer be isset(). If you use isset() to test a variable that is set to NULL, it will return FALSE. Also note that the null character ("\0") is not equivalent to PHP's NULL constant.

If multiple parameters are passed in at one time, isset() will only return TRUE when all parameters are set. The calculation process is from left to right. If an unset variable is encountered in the middle, it will be immediately stop.

Returns TRUE if var exists and the value is not NULL, otherwise returns FALSE.

isset() Example

<?php<br/><br/>$var = &#39;&#39;;<br/><br/>// 结果为 TRUE，所以后边的文本将被打印出来。<br/>if (isset($var)) {<br/>    echo "This var is set so I will print.";<br/>}<br/><br/>// 在后边的例子中，我们将使用 var_dump 输出 isset() 的返回值。<br/>// the return value of isset().<br/><br/>$a = "test";<br/>$b = "anothertest";<br/><br/>var_dump(isset($a));      // TRUE<br/>var_dump(isset($a, $b)); // TRUE<br/><br/>unset ($a);<br/><br/>var_dump(isset($a));     // FALSE<br/>var_dump(isset($a, $b)); // FALSE<br/><br/>$foo = NULL;<br/>var_dump(isset($foo));   // FALSE<br/><br/>?><br/>

isset() can only be used with variables, because passing any other parameters will cause a parsing error. If you want to check whether a constant has been set, use the defined() function.

Because it is a language constructor rather than a function, it cannot be called by variable functions.

If isset() is used to check the inaccessible properties of the object, this overloaded method will be called if the __isset() method has been defined.

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