$text=file_get_contents("baza.txt");
$word="слово";
preg_match_all("/.*?$word.*?\\./im",$text,$p);
foreach($p[0] as $v) echo "<li>$v";

function search($_POST){
$text=file_get_contents("baza.txt");
$test = fread($text, $_POST);
preg_match_all("/.*?$_POST.*?\\./im",$text,$p);
foreach($p[0] as $v) echo "<li>$v";