Definition of constants and single double quotation marks in define in php

I. definition constant of define & lt ;? Phpdefine (& amp; 39; DATABASE & amp; 39;, & amp; 39; MYSQL & amp; 39;); define (& amp; 39; DATABASE_USER & amp; 39;, & amp; 39; ROOT & amp; 39;); define (& a 1. define definition constant problem Directly use the "DATABASE" constant, which represents "MYSQL ". $ Arr = array ('fruit' => 'apple', 'veggi' => 'carrot'); define ('fruit', 'veggi '); print $ arr ['fruit']; // apple print $ arr [fruit]; // carrot * Note: [] contains no quotation marks. Php will automatically parse the key value as a constant, and then find the corresponding constant in the memory, so it finds the fruit and then parses the key value. Then, find the constant in the memory and find the veggie. then, treat veggie as the real key value and find the corresponding value. Repeated quotation marks 2. single quotes error_reporting (E_ALL); $ arr = array ('fruit' => 'apple', 'veggi' => 'carrot '); define ('fruit', 'veggi'); print $ arr ['fruit']; echo'
'; // Apple print $ arr [fruit]; echo'
'; // Carrot print "hello $ arr [fruit]"; echo'
'; // Apple print "hello {$ arr [fruit]}"; echo'
'; // Hello carrot print "hello {$ arr ['fruit']}"; echo'
'; // Hello apple // print "Hello $ arr ['fruit']"; // error // print "hello $ _ GET ['foo']"; // There is an error print "Hello ". $ arr ['fruit']; // Hello apple Note: If no quotation marks are added to the array square brackets, php will resolve them by constants first.