Find the highest prime number in a given range

Question

I need to find the highest prime number in a given range.
Here is my code which works for 0-100 but if I give 0-125 it is showing prime number as 125.

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Answer 1

thanks for your reply Samuel Cook ..but i need without using 'gmp_nextprime()' function.. iamgetting message 'Call to undefined function gmp_intval() in C:\wamp\www\highprime.php on line 5' – user1659450 16 mins ago

Since you are having difficultly getting gmp functions to work then you can use

Example 1 : None GMP Function

$range = range(125, 0); // create the range
foreach ( $range as $v ) {
    if (isPrime($v)) {
        printf('highest prime number is %d', $v);
        break;
    }
}

If you are able to get gmp working then you can use gmp_prob_prime Function Used

Example 1 : Using gmp function

foreach ( $range as $v ) {
    if (gmp_prob_prime($v) == 2) {
        printf('highest prime number is %d', $v);
        break;
    }
}

Output

highest prime number is 113

Functions Used

function isPrime($num) {
    if ($num == 1)
        return false;

    if ($num == 2)
        return true;

    if ($num % 2 == 0) {
        return false;
    }

    for($i = 3; $i <= ceil(sqrt($num)); $i = $i + 2) {
        if ($num % $i == 0)
            return false;
    }
    return true;
}