Make an commercial price of original price with margin

commercialize.php

<?php

/**
 * Make nice commercial price from orignal price
 *
 * @param string   $price     Original price
 * @param string   $rangeDown Range down in percent
 * @param string   $rangeUp   Range up in percent
 * @param string   $minUnit   Minimal unit to increment up/down
 *
 * @return string Commercial price
 **/
function commercialize($price, $rangeDown = '10', $rangeUp = '0', $minUnit = '1.00')
{
	// Make price universal format, with two decimals
	// this allows str_replace later on	
	$price = bcmul($price, '1.00', 2);

	// First determine upper and lower boundaries
	$minPrice = bcsub($price, bcmul($price, bcdiv($rangeDown, '100', 3), 2));
	$maxPrice = bcadd($price, bcmul($price, bcdiv($rangeUp, '100', 3), 2));

	// Prices found in range
	$prices = [];

	// Go trough multiplication factors
	for($x = 9; $x >= 2; $x--) {
	
		// Base is used to substract one or add one to the current number to test
		$base = bcmul('1.00', bcmul($minUnit, bcpow('10.00', $x,  2)), 2);
		
		// Replace is used to replace everything after the base with 99
		$replace = bcmul(preg_replace('/0/', '9', str_replace('1', '', $base), $x), '1.00', 2);
		
		// Make sure auto added 0. is correctly handled
		if (strpos($replace, '0.') === 0) {
			$replace = substr($replace, 1);
		}
		
		// Create prices to test
		$testPrices = [
			$price,
			bcsub($price, $base, 2),
			bcadd($price, $base, 2)
		];
		
		// Test prices, and store them if the fall in range
		foreach($testPrices as $testPrice) {		
			$testPrice = substr($testPrice, 0, strlen($testPrice) - strlen($replace)) . $replace;			
			if (bccomp($testPrice, $maxPrice, 2) > 0) { continue; }			
			if (bccomp($minPrice, $testPrice, 2) > 0) { continue; }
			$prices[] = $testPrice;			
		}
	
	}
	
	// If no prices are found, just return the original
	if (count($prices) == 0) { return $price; }
	
	// Now test which price has the lowest difference with the 
	// original value
	$diff = str_replace('-', '', bcsub($price, $prices[0], 2));
	$out = $prices[0];	
	foreach($prices as $n_price) {
		$n_diff = str_replace('-', '', bcsub($price, $n_price, 2));
		if (bccomp($diff, $n_diff, 2) > 0) {
			$diff = $n_diff;
			$out = $n_price;
		}
	}
	
	return $out;
}

header("Content-type: text/plain");


$tests = [
	[ '12.21', '25', '25', '0.01' ], 
	[  '1518', '25', '25', '1.00' ], 
	[  '1684', '25', '25', '1.00' ], 
	[  '7193', '25', '25', '1.00' ], 
	[  '8018', '25', '25', '1.00' ], 
	[  '8.21', '25', '25', '0.01' ], 		
	[ '21.50', '25', '25', '0.01' ], 		
];

foreach($tests as $test) {
	list($price, $minRange, $maxRange, $minUnit) = $test;
	$commercial = commercialize($price, $minRange, $maxRange, $minUnit);	
	echo sprintf('%s --> %s', str_pad($price, 8, ' '), $commercial) . "\n";
}

// OUTPUT:
// 12.21    --> 11.99
// 1518     --> 1499.00
// 1684     --> 1699.00
// 7193     --> 7199.00
// 8018     --> 7999.00
// 8.21     --> 7.99
// 21.50    --> 21.99