Someone

<?php
//zch:	see the SOURCE CODE of the page generated with this
//		remember \n doesn't generates a carriage return in html ...

$search = "some text";
$str = sprintf("some text: some text some other text\nsome text: some text some other text\nsome text: some text some other text");
$tok = strtok($str, sprintf("\n"));

while($tok !== false){
	$text .= preg_replace("/^(" . $search . ")/", "<b>" . $search . "</b>", $tok);
	$text .= sprintf("\n");
	$tok = strtok("\n");
}

echo $text;

//echo preg_replace("/^(some text)/", "<b>some text</b>", $str);
?>