Calculates the amount in the invoice and returns a new amount with discount and tax added, the total also will be rounded to 2 decimal places. Discount and tax will only be applied when available.

calc_invoice.php

<?php
/**
 * Calculates the amount in the invoice and returns a new amount with discount and tax added, 
 * the total also will be rounded to 2 decimal places.  Discount and tax will only be applied 
 * when available.
 * 
 * USAGE: 
 * <code>
 *		// Example 1: Subtract discount from subtotal, add tax and return the grand-total.
 * 		calc_invoice( 500, 0.5, 5 );
 * 
 *		// Example 2: There's no discount, add tax to the subtotal and return the grand-total.
 * 		calc_invoice( 500, 0, 5 );
 * 
 *		// Example 3: There's a discount but tax was not provided. Return the subtotal minus the discount as a grand-total.
 * 		calc_invoice( 500, 0.5, 0 );
 * 
 *		// Example 4: There's no discount and tax. Return the subtotal as a grand-total.
 * 		calc_invoice( 500, 0, 0 );
 * </code>
 * 
 * Note: to print the result you can set the fourth param to true.
 * 
 * 
 * @param float|int $subtotal The total amount of the invoice fields.
 * @param float|int $discount The discount applied to the amount in the invoice fields.
 * @param float|int $tax The tax applied to the amount in the invoice fields.
 * @param bool $echo Whether to return or print the result.
 * @return float|int $total The calculation of the total amount in the invoice including discount and tax (if available).
 */
function calc_invoice( $subtotal, $discount, $tax, $echo = false ) {
	
	/* If there's a discount, subtract the discount from the subtotal. */
	$discount = $subtotal - ( $discount * ( $subtotal / 100 ) );
	
	/* Return the descount with precision. */
	$discount = round( $discount, 2 ); 
	
	/* Calculate the discount + the tax. */
	$total = $discount + ( $tax * ( $discount / 100 ) );
	
	/* Return or print the total after calculating the descount and the VAT. */
	if ( $echo == true )
		echo round( $total, 2 );
	else
		return round( $total, 2 );
}