I have got string response is "<?php $a=array("10","20"); echo $a[0]+a[1]; ?>"
How to execute this string. I use PHP 7.0.
Please give any solution.
As already stated you can use eval($string);. Beware that eval() evaluates a string as PHP code, so you need to be careful with the strings you allow to be passed!
If you take the string yourself from a predefined set of strings you created (using a swtich statement or something similar) it shouldn't be a problem.
If the string comes from the outside (user input or cURL requests or whatever) you shouldn't trust it and you should avoid using this method.