• PHP constants are identifier or name that can’t be changed during the execution of the script except for magic constants, which are not really constants.
• Constants can never be undefined or changed, they are similar to the variable except once defined.
• PHP constants follow the same PHP variable rules and it remains constant across the entire program.
• Conventionally, in uppercase letters PHP constants should be defined and it can be started with an underscore or letter only.
• PHP constants consists of two types, they are:
  • Using define () function
  • Using const keyword

Using define () function

• In PHP use the define () function to create a constant and it defines constant at run time.
• In this function name specifies the constant name, value specifies the constant value and case-insensitive specifies whether a constant is case-insensitive.
• By default, if value is false it means it is case sensitive.

Sample Code

<?php    
define("MESSAGE","Welcome to Wikitechy",true);//not case sensitive    
echo MESSAGE, "</br>";    
echo message;    
?>    

Output

Using const Keyword

• PHP introduces a const keyword to create a constant and it defines constants at compile time.
• It constructs a language, not a function.
• The constant defined using const keyword are case-sensitive.

Sample Code

<?php      
    define("MSG", "Wikitechy");  
    echo MSG, "</br>";  
    echo constant("MSG");  
    //both are similar  
?>  

Output