$result_txt = preg_replace("# программ(.+?) #is", " <b>программ\\1</b> ", $txt);

$txt = " ".$txt." ";
$txtl = strtolower($txt);
$len = strlen($txtl);
$start = array();
$finish = array();
for($i=0;$i<$len;$i++)
{
	if( substr($txtl,$i,9) == " программ" )
	{
		$start[$i] = $i;
		$pos = $i+9;
		while( substr($txtl,$pos,1) != " " and $pos <= $len )$pos++;
		if($pos < $len)$finish[$i] = $pos;
		else unset($start[$i]);
	}
}
$result_txt = "";
for($i=0;$i<$len;$i++)
{
	if( in_array($i,$start) )$result_txt .= "<b>";
	$result_txt .= substr($txt,$i,1);
	if( in_array($i,$finish) )$result_txt .= "</b>";
}
$result_txt = substr($result_txt,1,-1);