voldant

<?php
$a = [
'color' => ['red', 'blue', 'orange'],
'size' => ['10-12', '12-14', '13-13'],
'somekey' => ['val1', 'val2', 'val3'],
];

function addValue(&$arr, $key, $val)
{
	if (!count($arr)) {
		$arr[] = [$key => $val];
		return;
	}
	foreach($arr as $num => $values) {
		$arr[$num][$key] = $val;	
	}
	
}

$b = [];

foreach($a as $key => $values) {
	
	$copyB = $b;
	$newB = [];
	foreach($values as $value) {
		$currentB = $copyB;
		addValue($currentB, $key, $value);
		$newB = array_merge($newB, $currentB);
	}
	$b = $newB;
	
}

print_r($b);