An example of Isset () function variable in PHP

The Isset function is to detect whether a variable is set.

Format: bool Isset (mixed var [, mixed Var [, ...]])

return value:

Returns FALSE if the variable does not exist
Returns FALSE if the variable exists and its value is null
Returns ture if the variable exists and the value is not NULL
When you check multiple variables at the same time, each item returns TRUE when it meets the previous requirement, otherwise the result is FALSE
If you have freed a variable using unset (), it will no longer be isset (). If you use Isset () to test a variable that is set to NULL, it returns FALSE. Also note that a null byte ("") is not equivalent to PHP's null constant.

To get user properties, some users can, some users cannot, the same logic why can not be checked using code like this

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$userInfo = ' abc '; if (Isset ($userInfo [' account '])) { $account = $userInfo [' account ']; } else { $account = $userInfo; }

Found $account was empty, checked some data, and didn't get the results.
Did an experiment to get the conclusion

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$a = ' abc '; echo $a [' account '];

The result is a.

When the string key is not there, it translates into a number key, which is 0, so it gets a
Now the php5.4 has been repaired.

Take this opportunity to tidy up the use of isset, for everyone to learn from

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<?php $var = '; if (Isset ($var)) { Print "This var is set set and so I'll print."; } In the example below, we will use the Var_dump function to output the return value of Isset (). $a = "Test"; $b = "Anothertest"; Var_dump (Isset ($a)); TRUE Var_dump (Isset ($a, $b)); TRUE unset ($a); Var_dump (Isset ($a)); FALSE Var_dump (Isset ($a, $b)); FALSE $foo = NULL; Var_dump (Isset ($foo)); FALSE ?>

This is also valid for elements in the array:

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<?php $a = array (' Test ' => 1, ' hello ' => NULL); Var_dump (isset ($a [' test ')];//TRUE Var_dump (isset ($a [' foo ')];//FALSE Var_dump (isset ($a [' hello '));//FALSE ' Hello ' equals NULL, so it is considered unassigned. If you want to detect NULL key values, try the bottom method. Var_dump (array_key_exists (' hello ', $a)); TRUE ?>

The value of the key ' B ' is equal to NULL, so it is considered an open value.
But the value of the key ' C ' is null. The result is true, and the null value is set. If you want to detect NULL key values, try the bottom method.

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Var_dump (array_key_exists (' hello ', $a)); TRUE

Warning: isset () can only be used for variables because passing any other parameter will result in a parse error. To detect if a constant is set, use the defined () function.

Some examples of applications

PHP isset () for PHP parameters you can use the PHP isset () to determine whether a parameter is defined, note that if the parameter is empty, or "n" (null byte) after using PHP isset () to judge, will be true.

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<?php $weigeti = '; Here the initial parameter $weigeti= '; $weigeti 0=false//Here The initial parameter is Boolean FALSE $weigeti 2=null; Here defines $weigeti2=null, returns false with Isset () judgement $weigeti 3 = ""; Here is a null byte, judged by Isset (), returns True, different from the null constant $weigeti 4= "www.v-get.com"; The definition here will be unset down. if (Isset ($weigeti)) {echo ' parameter $weigeti is set and the value is not NULL ';} Parameter $weigeti has been assigned and the value is $weigeti= ', so the parameter $weigeti has been set. if (Isset ($weigeti 0)) {echo ' parameter $weigeti0 has been set and the value is not NULL ';} Parameter $weigeti0 has been assigned and the value is $weigeti=false, so the parameter $weigeti is set. if (Isset ($vget)) {echo ' parameter $vget is set and the value is not NULL ';} Parameter $vget is not mentioned at all, Isset ($vget) returns false and does not output if (Isset ($weigeti 2)) {echo ' parameter $weigeti2 has been set and the value is not NULL ';} This is $WEIGETI2, although it is set, but $weigeti2=null, so Isset ($weigeti 2) returns false and does not output if (Isset ($weigeti 3)) {echo ' parameter $weigeti3 has been set and the value is not NULL ';} Parameter $weigeti3, although it represents a null byte, is not a constant null, so $isset ("") returns True, outputting Unset ($weigeti 4); It's released to $weigeti4. if (Isset ($weigeti 4)) {echo ' parameter $weigeti4 has been set and the value is not NULL ';} Although the beginning has been defined $weigeti4, and is not NULL, but then to unset () dropped, so will not output. ?>

PHP isset () in the use of PHP arrays, you need to determine whether a value of the array exists or use the PHP isset () function.

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<?php $V =array ( ' Weigeti ' => ' www.111cn.net ', ' V-get ' =>array (     ' www ' => ') Www.v-get.com ',     ' e ' => ' e.v-get.com ',     ' Wuliu ' =>array (          ' www ' => ' wuliu.v-get.com ',         ' Yiwu ' => ' www.111cn.net '     )   ) ); //Var_dump for output true or FALSE Var_dump (isset ($V [' Weigeti ']); //array $v $v[' weigeti '] = ' www.111cn.net ' already exists, so output true Var_dump ( Isset ($V [' v-get '] [' e '])]; //$V [' v-get '] [' e ']= ' e.v-get.com ' so outputs TRUE Var_dump (Isset ($V [' v-get '] [' Wuliu '] [' Yiwu ']); //$V [' v-get '] [' Wuliu '] [' Yiwu ']= ' www.111cn.net ' so outputs true Var_dump (Isset ($V [' v-get '] [' Wuliu '] [' sh ']]); //Based on array, the item is not set at all, so output is false

PHP isset () multi-parameter judgement many times we need to judge multiple parameters, we can use Isset ($A) &&isset ($B) ... To determine if all of these parameters have been set and are NOT NULL, you can use Isset () multiple parameters to determine if it is all set.

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<?php Isset ($v 1) &&isset ($v 2) &&isset ($v 3) ... Equivalent to Isset ($v 1, $v 2, $v 3 ...) ?>

Using Isset () to determine multiple parameters, all parameters are set and are not NULL, so that the entire isset () is false as long as one of the parameters is not set or null.

PHP isset () judges $_post, $_get, $_request equivalents. Using Isset () or empty () to determine the parameters passed through the form is the most common use of isset ().

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if (Isset ($_post[' from ']) && ' e.v-get.com ' ==$_post[' from ') { The source of the Echo ' website via post is e.v-get.com '; }

According to the above we can use Isset to judge multiple parameters, in the form passed the value of the most convenient to judge.

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<?php Form delivery The user name and pass must exist at the same time to perform if (Isset ($_post[' user '],$_post[' pass ')) { $user =$_post[' user ']; $pass =$_post[' Pass ']; Echo ' The username you fill out is: ', $user, '; The password is: ', $pass; } ?>

PHP isset ($var {string length}) to determine the length of a string in PHP to determine whether a string exists or to determine whether the string length exceeds a number or whether the string length is null general use strlen (), but in fact, the use of isset () performance is superior.

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<?php //ternary, equivalent to if (Isset ($_post[' from ')) {$weigeti =$_post[' from '];} else{$weiget = ';} $weigeti =isset ($_post[' from ') "$_post[' from ']:"; //Judge $weigeti exists with string length greater than 0 /* Rookie writing   if (strlen ($weigeti) >0) { echo ' $weigeti substring longer than 0 ';} */ //Better performance to determine whether the $weigeti 1th character is set if (Isset ($weigeti {1})) { echo ' $weigeti character length greater than 0 '; } ?