PHP null value judgment

When using PHP to write page programs, I often use the variable processing function to determine whether the variable value of a parameter at the end of the PHP page is null. At the beginning, I used to use the empty () function, but some problems are found. Therefore, the isset () function is used instead.

As the name suggests, empty determines whether a variable is "null" and isset determines whether a variable has been set. This so-called "name implies" makes me take some detours at the beginning: When a variable value is equal to 0, empty () will also be true ), as a result, some accidents may occur. Originally, although empty and isset are both variable processing functions, they are used to determine whether the variables have been configured, but they are different: Empty also checks whether the variables are empty and zero. When the value of a variable is 0, empty considers this variable to be null, that is, it is equivalent to not being set.

For example, if the $ id variable is detected and $ id = 0, empty and isset are used to check whether the variable $ ID has been configured. Different values will be returned for both -- empty considers no configuration, isset can get the value of $ ID:

$ Id = 0;
Empty ($ id )? Print "It's empty.": Print "it's $ id .";
// Result: it's empty.
Print "<br> ";
! Isset ($ id )? Print "It's empty.": Print "it's $ id .";
// Result: It's 0.

This means that when we use a variable to process a function, when the variable may have a value of 0, we should be careful when using empty. In this case, it is more wise to replace it with isset.

When the URL tail parameter of a PHP page appears (for example, test. php? Id = 0), try to compare:

If (empty ($ id) $ id = 1;-if the ID is 1
If (! Isset ($ id) $ id = 1;-if the ID is not 1

Run the following code separately to detect the above inference:

If (empty ($ id) $ id = 1;
Print $ ID; // get 1

If (! Isset ($ id) $ id = 1;
Print $ ID; // get 0

This article from the csdn blog, reproduced please indicate the source: http://blog.csdn.net/akailee/archive/2009/04/23/4104283.aspx