Calculate the next increment value of a string of numbers and letters

StringCalc.php

<?php

	$testCases[] = '123';    // Should be 124
	$testCases[] = '123=RH'; // Should be 124=RH
	$testCases[] = '123RH';  // Should be 124RH
	$testCases[] = '123R4';  // Should be 124R4
	$testCases[] = 'RH123';  // Should be RH124
	$testCases[] = 'RH-123'; // Should be RH-124

	foreach($testCases as $testCase) {
		$orderNumber = extractNumbersFromString($testCase);
		$orderPrefix = extractOrderPrefix($testCase, $orderNumber);
		print(incrementOrderNumber($orderNumber, $orderPrefix) . "\n");
	}

	function extractNumbersFromString($string) {
		preg_match_all('!\d+!', $string, $matches);
		return $matches[0][0];
	}

	function incrementOrderNumber($order_number, $prefix) {
		$order_number = ( (int) $order_number ) + 1;
		if($prefix[0] !== '') {
			return $prefix[0] . $order_number;
		}
		if($prefix[1] !== '') {
			return $order_number . $prefix[1];
		}
		return (string) $order_number;
	}

	function extractOrderPrefix($string, $orderNumber) {
		return explode($orderNumber, $string);
	}