Examples of differences and usages of define and defined functions in PHP

Define

define is used to define a constant, which is also global in scope. You can access it anywhere in the script without the scope of the control. Once a constant is defined, it can no longer be changed or undefined.

There are a lot of similarities between constants and variables, so it's easy to confuse; Let's look at the differences between constants and variables:

1. A constant value cannot be changed after it has been specified

2. When you set a constant, you do not need to precede the "$" symbol

3. Constants can be accessed by domains of all scopes

4. The value of a constant can only be "strings [string]" and "numbers [number]"

The syntax format for define is as follows:

Define (name,value,case_insensitive)

parameter
name
value
case_insensitive

Specify a constant (case-sensitive) with the sample code as follows:

<?phpdefine ("greeting", "How is you today?"); ECHO constant ("greeting");? >

Output the following result:

How is you today?

Specify a constant (case-insensitive) with the sample code as follows:

<?phpdefine ("greeting", "How is you today?", TRUE); Echo constant ("greeting"); >

Output the following result:

How is you today?

Defined

The defined is used to detect that a constant is undefined and returns true if the constant exists, otherwise false.

The syntax format for defined is as follows:

Defined (name)

Parameters	Description
Name	Necessary parameters. Specifies the name of a constant object

The sample code is as follows:

<?phpdefine ("greeting", "How is you today?"); echo defined ("greeting");? >

Output the following results:1

The defined tag of the thinkphp template engine is used to determine whether a constant has been defined.

Defined tag usage is as follows:

<present name= "Constants" > What to Output </present>

Use the defined tag in the template/tpl/default/index/display.html as follows:

<defined name= "site_name" > website name:{*site_name}</defined>

If the judgment is not defined, you can use:

<notdefined name= "Site_Name" >{*site_name} does not exist live undefined </notdefined>