PHP区分大小写：函数名和类名不区分,变量名区分

<p><?php</p><p>$abc = 'abcd';</p><p>echo $abc; //输出 'abcd'</p><p><span>立即学习</span>“<a href="https://pan.quark.cn/s/7fc7563c4182" style="text-decoration: underline !important; color: blue; font-weight: bolder;" rel="nofollow" target="_blank">PHP免费学习笔记（深入）</a>”；</p><p>echo $aBc; //无输出</p><p>echo $ABC; //无输出</p>

<p><?php</p><p>define("ABC","Hello World");</p><p>echo ABC; //输出 Hello World</p><p>echo abc; //输出 abc</p>

<p><?php</p><p>function show(){</p><p>echo "Hello World";</p><p> }</p><p>show(); //输出 Hello World 推荐写法</p><p>SHOW(); //输出 Hello World</p>

<p><?php</p><p>class cls{</p><p>static function func(){</p><p>echo "hello world";</p><p>}</p><p>}</p><p>Cls::FunC(); //输出hello world</p>

<p><?php</p><p>echo __line__; //输出 2</p><p>echo __LINE__; //输出 3</p>

<p><?php</p><p>$a = null;</p><p>$b = NULL;</p><p>$c = true;</p><p>$d = TRUE;</p><p>$e = false;</p><p>$f = FALSE;</p><p>var_dump($a == $b); //输出 boolean true</p><p>var_dump($c == $d); //输出 boolean true</p><p>var_dump($e == $f); //输出 boolean true</p>