Constants are same like variables but once constants values are defined it can not be changed or undefined. Constants are used to define settings/configurations which remains same through out the execution. actually constants are used when you want a surety that these values will not change in throughout program execution cycle.
Define constant is very simple like define("constant name","Value");
<!doctype html> <html> <head> <title> PHP Constants </title> </head> <body> <?php define("WELCOME","Welcome to PHPDocs.com!"); echo WELCOME; //this is will output Welcome to PHPDocs.com! ?> </body> </html>