Simple Algorigthm: Closest biggest adjacent number

adjacent.php

<?php
// By Carlos Gratacos

// Input: 1 interger number
// Output: false-> if not possible, else return return the smaller or greater input by switching adjacent digit
// Biggest closest to the original one

// 123 -> 132
// 
// 254 -> 524
//
// 1653 -> (1635, 1653) -> 6135
// 
// 71132 -> 71312
// 
// 9876543210 -> false
// 

function biggest(int $x) {	
	$str_num = (string)$x;
	
	$biggest = rotation($str_num);
	
	return $biggest > $x ? $biggest: false; 
		
}


// Rotate
function rotation(string $num) {
	$arr = str_split($num);
	$stop = null;
	for($i = strlen($num)-1; $i > 0; $i-- ) {
		if ($arr[$i] > $arr[$i-1]) {
			$last = $arr[$i];
			$arr[$i] = $arr[$i-1];
			$arr[$i-1] = $last;
			$stop = $i;
			break;
		}
	}
	
	if(isset($stop)){
		for($i = $stop; $i < strlen($num)-1; $i++ ) {
			if ($arr[$i] > $arr[$i+1]) {
				$last = $arr[$i];
				$arr[$i] = $arr[$i+1];
				$arr[$i+1] = $last;
			}
		}
	}
	return (int)implode('', $arr);
}


function test(){
	$arr = [123,254, 1653, 0, 71132,45325, 9876543210];
	
	foreach($arr as $val) {
		$x =biggest($val);
		echo $val." biggest number is " . ($x?:"None") . "<br>";
	}
}

test();