When double quotes are used to assign values to string variables, the PHP interpreter must check the entire string to see if it contains any variables that need to be inserted into the string. This process is called:

Interpolation string.

Explanation:

The proces of adding the variable in middle the string.The interpolation  Replace the value of the Variable when the string data is processed .In the Interpolation process  the variable is permitted with string containing double quotes.In the interpolation string  the interpreter check the entirely string that  the variable in middle the string.

Following is the example of interpolation string in Php

$n1 = "RAM";

echo "hello $name"; // output

Output:

hello RAM