ทำไฮไลท์ข้อความที่ค้นหาด้วย str_replace แต่ข้อความที่รับมาเป็น array จะนำไปใช้อย่างไรครับ

$a = array(
		    'A' => 'a',
		    'M'=>'m',
		    'P' => 'php' 
		    );
$keySearch= $a['P'];
$myStrong = "Welcome to my php programming";
echo str_replace($keySearch,"<font color=red><b>$keySearch<b></font>",$myStrong);

$a = array(
		    'A' => 'Wel',
		    'M'=>'my',
		    'P' => 'php' 
		    );
$keySearch= $a['P'];
$myStrong = "Welcome to my php programming";
echo str_replace($keySearch,"<font color=red><b>$keySearch<b></font>",$myStrong);

$a = array('Wel', 'my', 'php');
$b = array();
foreach($a as $val){
	$b[$val] = '<b>'.$val.'</b>';
}
$myStrong = "Welcome to my php programming";
echo str_replace(array_keys($b), $b,$myStrong);