How can a PHP program determine the number of rows in a query result?

To determine the number of rows in a query result in PHP, you can use the `mysqli_num_rows()` function. This function takes a result set object as its argument and returns the number of rows in the result set.

Here's an example:

// Connect to database
$conn = mysqli_connect("localhost", "username", "password", "database");

// Execute query
$result = mysqli_query($conn, "SELECT * FROM mytable");

// Get number of rows
$num_rows = mysqli_num_rows($result);

// Print number of rows
echo "Number of rows: " . $num_rows;

This code connects to a MySQL database, executes a SELECT query, and then uses `mysqli_num_rows()` to get the number of rows in the result set. The number of rows is then printed to the screen.