PHP define and defined

PHP define is used to define constants, defined is used to determine whether constants are defined, but there is a small trap in it ... Let's look at the following example:

 
    
Var_dump (defined (newdb));
   
Output:bool (True)
   
   
Var_dump (Constant (' defaultdbname '));//Output:string (4) "prod"//correct usage if (defined (' Defaultdbname ')) {    $db = Defaultdbname;    $db = constant (' defaultdbname ')}//error usage if (defined (defaultdbname)) {    //The consequences are serious: The following will not be performed, do not believe you try    $db = Defaultdbname;    $db = constant (' defaultdbname ')}?>
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