How to make a file stream variable run

How do i make a file stream variable run?
1. text file a.txt, it says $ A
2. I have a my.php, which is written in this way.

 
 $a = "OK";
File_get_content ("A.txt", "r+");

This is the operation code, I will not
The file reads in, I'm going to save the text again, but I want to replace $ A in the text with a $ A value in PHP. OK

File_put_content ("B.txt", "w+");

Text read into PHP, the inside of the things do not seem to react with PHP, Open is $ A, in PHP ran a circle to save or $ A, there is no way to let the file flow inside the variables in PHP run it?

that contains $ A a.txt-----> read PHP--------> contains OK a.txt

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------Solution--------------------
Text substitution
OB + eval
OB + include
OB + Phar

------Solution--------------------
Take a look at the string function.
------Solution--------------------
Presumably guess you need to use Eval for this function

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