Как найти строку в файле в php?

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$file = file_get_contents('example.txt');
$search = 'example';
$position = strpos($file, $search);
if ($position !== false) {
    echo "The string '$search' was found in the file at position $position.";
} else {
    echo "The string '$search' was not found in the file.";
}

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$file = file_get_contents('example.txt');
$search = '/example/';
preg_match_all($search, $file, $matches);
print_r($matches);

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$lines = file('example.txt');
$search = 'example';
$result = array_filter($lines, function($var) use ($search) { return preg_match("/$search/", $var); });
print_r($result);