Why does PHP have int and float data types?

I have realized that PHP can treat string variables as numbers (as long as int or float values are stored in the string variables), for example:

<?php
    // Declaring two int numbers and one float number as string variables
    $i = "10";
    $j = "3";
    $f = "5.2";
    
    // Adding an int number and a float number represented as string variables
    $result = $i + $f;
    echo $result . "<br />"; // will print "15.2"
    
    // Comparing two int numbers represented as string variables
    if ($i > $j)
    {
        echo "i is bigger than j"; // this statement will be executed
    }
    else
    {
        echo "i is not bigger than j";
    }
?>

My question is, why do we have int and float data types in PHP, why not just store int and float numbers in string variables like I did in the above code?