cat /tmp/test
qwe
йцу

<?php
$content_log = shell_exec("grep -w 'qw' /tmp/test");	
$content_log1 = shell_exec("grep -w 'йц' /tmp/test");	

var_dump($content_log);
echo "</br>";
var_dump($content_log1);
?>

NULL 
string(7) "йцу "

01:05:09 n84689 ~ $
01:05:09 n84689 ~ $ cat /tmp/a.php
<?php
$content_log = shell_exec("grep -w 'йц' /tmp/test");
var_dump($content_log);
?>
01:05:15 n84689 ~ $ cat /tmp/test
йцу
01:05:22 n84689 ~ $ grep -w 'йц' /tmp/test
01:05:35 n84689 ~ $ /usr/bin/php /tmp/a.php
NULL
01:05:40 n84689 ~ $