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<?php
$string = "radar";
if ($string == strrev($string)) {
    echo "$string is a palindrome";
} else {
    echo "$string is not a palindrome";
}
?>

<?php
function is_prime($n) {
    if ($n <= 1) {
        return false;
    }
    for ($i = 2; $i <= sqrt($n); $i++) {
        if ($n % $i == 0) {
            return false;
        }
    }
    return true;
}

$num = 7;
if (is_prime($num)) {
    echo "$num is a prime number";
} else {
    echo "$num is not a prime number";
}
?>

<?php
function longest_palindromic_substring($str) {
    $max_length = 0;
    $start = 0;
    $length = strlen($str);

    for ($i = 0; $i < $length; $i++) {
        for ($j = $i; $j < $length; $j++) {
            $substring = substr($str, $i, $j - $i + 1);
            if ($j - $i + 1 > $max_length && $substring === strrev($substring)) {
                $max_length = $j - $i + 1;
                $start = $i;
            }
        }
    }

    return substr($str, $start, $max_length);
}

$str = "abacdfgdcaba";
echo "Longest palindromic substring in \"$str\": " . longest_palindromic_substring($str);
?>