Okay here's our code of this captcha image:

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<?php
header('Content-type: image/jpg');
$im = imagecreatefromjpeg ("image.jpg");
$rand1 = rand(1,255);
$rand2 = rand(1,255);
$rand3 = rand(1,255);
$color = imagecolorallocate($im, $rand1, $rand2, $rand3);
$text = rand(1000, 9999);
$font = 'text-font.otf';
$size = 30;
imagettftext($im, $size, 0, 70, 45, $color, $font, $text);
imagejpeg($im);
imagedestroy($im);
?>

But if we want to use this image on our sites we must add:

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$_SESSION['captcha'] = $text;

on the bottom of our captcha file. So.. now let's make our form secure.

First of all we must start the session

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<?php
session_start();

session_start(); - Starts the session

Now I'm using a form for checking that is the security code correct

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if(isset($_POST['submit'])){
$ses = $_SESSION['captcha'];
if($ses == $_POST['code']){
echo "Code is correct!";
}else{
echo "Code is incorrect!";
}

if(isset($_POST['submit']) && !empty($_POST['code'])){ - If someone has clicked the submit button let's continue with our script
$ses = $_SESSION['captcha']; - Let's get the captcha code from the session
if($ses == $_POST['code']){ - If captcha's code is same as the code you posted
echo "Code is correct!"; - let's say that the code was correct
}else{ - but if they are not same
echo "Code is incorrect!"; - let's say that the code is not correct
} - This ends the if and else statement

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}else{
echo "<img src='captcha.php'><br>";
echo "<form action='' method='post'>"
    ."<input type='text' name='code'>"
    ."<input type='submit' name='submit' value='Enter'>"
    ."</form>";
}

}else{ - If anyone hasnt clicked the submit button let's display our form
echo "<img src='captcha.php'><br>"; - Shows our captcha image
echo "<form action='' method='post'>"
."<input type='text' name='code'>"
."<input type='submit' name='submit' value='Enter'>"
."</form>"; - Our form
} - This ends the if and else statement

code:%20select%20all%20

<?phpsession_start();$im%20=%20imagecreatefromjpeg ("image.jpg");
$rand1 = rand(1,255);
$rand2 = rand(1,255);
$rand3 = rand(1,255);
$color = imagecolorallocate($im, $rand1, $rand2, $rand3);
$text = rand(1000, 9999);
$font = 'text-font.otf';
$size = 30;
imagettftext($im, $size, 0, 70, 45, $color, $font, $text);
header('Content-type: image/jpg');
imagejpeg($im);
imagedestroy($im);
$_SESSION['captcha'] = $text;
?>

index.php

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<?php
session_start();

if(isset($_POST['submit']) && !empty($_POST['code'])){
$ses = $_SESSION['captcha'];
if($ses == $_POST['code']){
echo "Code is correct!";
}else{
echo "Code is incorrect!";
}
}else{
echo "<img src='captcha.php'><br>";
echo "<form action='' method='post'>"
    ."<input type='text' name='code'>"
    ."<input type='submit' name='submit' value='Enter'>"
    ."</form>";
}
?>

Enjoy!

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