";     /*unset：释放变量*/     $num2 = 10;     unset($num2);     if(isset($num2)){
           echo"num2被定义了";     }else{
           echo"num2没有被定义";     } echo"
";     /*empty      * 判断是否为空*/     $num3 = ' ';     if(empty($num3)){
           echo"num3是空";     }else{
           echo"num不是空";     }     /*变量名大小写敏感*/     $name = 'zhangsan';     $NAME = 'lisi';     echo$name;     echo"
";     echo$NAME; echo"
";     /*单引号里面全是字符串      * 双引号可解析变量*/     $net = "lemon";     $age = 18;     /*双引号套单引号要加转移符*/     $string = "今天天气不错{$age}岁的{$net}出去\"玩\"了";     /*单引号套单引号要加转移符*/     $string2 = '今天天气不错{$age}岁的{$net}出\'去\'玩了';     echo$string; echo"
";     echo$string2; echo"
";     /*两种拼接字符串的方法*/     $string4 = '今天天气晴朗';     $string5 = '我们出去玩吧';     $result = "{$string4}{$string5}";     echo$result; echo"
";     echo$string4.$string5;