This is a sample code from the book I am reading:
ob_start();
include("{$path}.ini");
$string = ob_get_contents();
ob_end_clean();
$pairs = parse_ini_string($string);
My question is, how does ob_get_contents() know what to get contents from? ({$path}.ini in this situation)?
ob_get_contents simply gets the contents of the output buffer since you called ob_start(). Essentially, an output buffer in PHP catches anything that would have been output to the browser (excluding headers). It's useful in cases where you may need to filter some output, or you're using a PHP method (such as var_dump) that writes output directly to screen, and you would instead like the return value of the method in a string.
In this case, because you're include()ing the .ini file, it's contents will be essentially output to screen, and ob_get_contents() will get the content of the file.
If you were to put echo "I'm a little teapot short and stout"; underneath the include, this would also be included in $string after the body of the .ini file.
In your specific case, however, output buffering is an unnecessary overhead, simply use file_get_contents on the .ini file. I'm not sure why a book would even have this code in it at all.