How do I convert a string to a number in PHP?

You don’t typically need to do this, since PHP will coerce the type for you in most circumstances. For situations where you do want to explicitly convert the type, cast it:
$num = “3.14”;
$int = (int)$num;
$float = (float)$num;

There are a few ways to do so:

Cast the strings to numeric primitive data types:
$num = (int) “10”;
$num = (double) “10.12”; // same as (float) “10.12”;

Perform math operations on the strings:
$num = “10” + 1;
$num = floor(“10.1”);

Use intval() or floatval():
$num = intval(“10”);
$num = floatval(“10.1”);

Use settype().