In PHP, define defines constants and single double quotes.

First, define define constant issues        <?php   define (' DATABASE ', ' MYSQL ');   Define (' Database_user ', ' ROOT ');   Define (' Database_password ', ' PASSWORD ');  ?>                 Direct use of "DATABASE" constant, represents "MYSQL".                 $arr = array (' Fruit ' => ' apple ', ' veggie ' => ' carrot ');                define (' fruit ', ' veggie ');                            print $arr [' Fruit '];  // apple              print $arr [fruit];  // carrot               * Note: There are no quotes in []. PHP automatically resolves the key value as a constant, then goes to the memory to find the corresponding constant, and then finds the fruit and then resolves the key value. Then go to the memory to find the constant, did not find veggie, and then the veggie as a real key value, find the corresponding value.                 second, single double quotes problem       error_reporting (E_all);       $arr = Array (' Fruit ' => ' apple ', ' veggie ' => ' carrot ');       define (' fruit ', ' veggie ');     Print $arr [' fruit ']; Echo ' <br/> ';    //apple     print $arr [fruit]; Echo ' <br/> ';    //carrot     print "Hello $arr [fruit]"; Echo ' <br/> ';  //Apple     print "Hello {$arr [fruit]}"; Echo ' <br/> '; Hello Carrot     print "Hello {$arr [' Fruit ']}";  echo ' <br/> '/hello Apple    //print "Hello $arr [' fruit ']"; There are errors    //print "Hello $_get[' foo ']"; There are errors    print "Hello". $arr [' Fruit ']; Hello Apple   Note: If the array square brackets are not enclosed in quotes, PHP will first parse by constants.