<?php 
echo checkWord('aabbc','abaac').PHP_EOL;
echo checkWord('funday','sunday').PHP_EOL;
echo checkWord('flipper','ripple').PHP_EOL;
echo checkWord('monkey','kenney').PHP_EOL;

function checkWord($guess, $solution){
	$arr1 = str_split($solution);
	$arr2 = str_split($guess);
	$arr1_c = array_count_values($arr1);
	$arr2_c = array_count_values($arr2);
	$out = '';
	foreach($arr2 as $key=>$value){
		$arr1_c[$value]=(isset($arr1_c[$value])?$arr1_c[$value]-1:0);
		$arr2_c[$value]=(isset($arr2_c[$value])?$arr2_c[$value]-1:0);

		if(isset($arr2[$key]) && isset($arr1[$key]) && $arr1[$key] == $arr2[$key]){
			$out .='<span style="color:green;">'.$arr2[$key].'</span>';
		}elseif(in_array($value,$arr1) && $arr2_c[$value] >= 0 && $arr1_c[$value] >= 0){
			$out .='<span style="color:red;">'.$arr2[$key].'</span>';
		}else{
			$out .='<span style="color:black;">'.$arr2[$key].'</span>';
		}
	}
	return $out;
}
?>

<span style="color:green;">a</span><span style="color:red;">a</span><span style="color:red;">b</span><span style="color:black;">b</span><span style="color:green;">c</span>
<span style="color:black;">f</span><span style="color:green;">u</span><span style="color:green;">n</span><span style="color:green;">d</span><span style="color:green;">a</span><span style="color:green;">y</span>
<span style="color:black;">f</span><span style="color:red;">l</span><span style="color:red;">i</span><span style="color:green;">p</span><span style="color:red;">p</span><span style="color:green;">e</span><span style="color:red;">r</span>
<span style="color:black;">m</span><span style="color:black;">o</span><span style="color:green;">n</span><span style="color:red;">k</span><span style="color:green;">e</span><span style="color:green;">y</span>