PHP double quotation marks to explain the security issues

PHP single quotes and double quotes:
The only difference between single quotes and double quotes is:
The PHP interpreter wrapped in single quotes will not explain the second, but double quotes will be explained twice,
Check the Code:
 
 
 
 
Thank you, Y0UMER.
 
 
In this way, we know the difference between single quotes and double quotes, but this will bring a security vulnerability.
 
If it is executed, it means that the string variables directly assigned values should be enclosed in single quotes.
How about array?
 
The array is successfully parsed, which means this:
As a configuration file, do not use double quotation marks as much as possible, so that it is easy to be intruded or maliciously attacked.