$hello= 3;
echo "hello is $hello"; // 打印结果:hello is 3 
echo 'hello is $hello'; // 打印结果: hello is $hello 
echo "hello is $hello\n"; // 打印结果: hello is 2 (同时换行) 
echo 'hello is $hello\n'; // 打印结果: hello is $hello\n

echo "This is a string"; 

ECHO    'This is a string' 

echo 'This is a string'; 

 ECHO    'This is a string' 

echo "This is a $variable"; 

INIT STRING ~0 

2 ADD_STRING  ~0 ~0 'This' 

3 ADD_STRING  ~0 ~0 ' ' 

4 ADD_STRING  ~0 ~0 'is' 

5 ADD_STRING  ~0 ~0 ' ' 

6 ADD_STRING  ~0 ~0 'a' 

7 ADD_STRING  ~0 ~0 ' ' 

8 ADD_VAR    ~0 ~0 !0 

9 ECHO      ~0 

echo "This is a " . $variable; 

CONCAT    ~0 'This is a ' !0 

2 ECHO      ~0