Captcha is working fine on localhost , but not online

I’m using simple-php-captcha( https://github.com/claviska/simple-php-captcha ) script on an ajax form in my wordpress theme,
it works fine when it’s on localhost but when i upload it on an online host , the captcha codes don’t match , every thing works fine , captcha image loads , sessions get created but the captcha code displayed in the image is not the same as when the form is submitted.

functions.php

require_once( get_template_directory() . '/libs/captcha/simple-php-captcha.php' );
require_once( get_template_directory() . '/inc/ajax/testimonial.php' );

header.php

session_start();
$_SESSION['captcha'] = simple_php_captcha(); 

The html form

<form action="<?php echo admin_url("admin-ajax.php"); ?>" class="dw-ajax-form dw-form" method="post" id="send_testimonial">
    <input type="text" name="name" placeholder="name">
    <input type="text" name="job" placeholder="company / job">
    <input type="text" name="email" placeholder="Email address">
    <textarea type="textarea" name="comment" placeholder="your opinion about us"></textarea>

    <div class="block captcha-image">
        <img src="<?php echo $_SESSION['captcha']["image_src"]; ?>" alt="<?php echo $_SESSION['captcha']["code"]; ?>">
    </div>

    <input type="text" name="captcha" placeholder="enter the code above" autocomplete="off">

    <input type="hidden" name="action" value="send_testimonial">
    <?php wp_nonce_field( 'send_testimonial', 'send_testimonial_nonce' ); ?>

    <input type="submit" value="send"> <span class="msg" style="margin-right:15px;"></span>
</form>

The ajax function ( /inc/ajax/testimonial.php )

<?php
/**
 * Testimonial Form Ajax Callbacks
 *
 * @package WordPress
 * @subpackage Learnfiles-shop Theme
 * @author Dornaweb.com
 */

add_action( 'wp_ajax_send_testimonial', 'dw_send_testimonial' );
add_action( 'wp_ajax_nopriv_send_testimonial', 'dw_send_testimonial' );
function dw_send_testimonial() {
    global $wpdb;
    $message = '';

    $name           = strip_tags( htmlspecialchars( $_POST["name"] ) );
    $job            = strip_tags( htmlspecialchars( $_POST["job"] ) );
    $email          = strip_tags( htmlspecialchars( $_POST["email"] ) );
    $comment        = strip_tags( htmlspecialchars( $_POST["comment"] ) );

    /* captcha */
    $captcha_input  = strtolower( strip_tags( htmlspecialchars( $_POST["captcha"] ) ) );
    $captcha_code = strtolower( $_SESSION['captcha']['code'] );

    /** Validation **/
    if( !$_SESSION['captcha'] || !is_array( $_SESSION['captcha'] ) )
        die( '<span class="error">Somethings wrong</span>' );

    /******************************* IT ALWAYS GIVES ME THIS ERROR WHEN ONLINE , BUT IT WORKS ON LOCALHOST( i also tried it with "!=" operator ) **************/
    if( $captcha_code !== $captcha_input )
        die( '<span class="error">The entered code doesnt match</span>' );
    /**********************************************************************************************************************************************************/

    if (  !isset( $_POST['send_testimonial_nonce'] ) || ! wp_verify_nonce( $_POST['send_testimonial_nonce'], 'send_testimonial' ) )
        die('<span class="error">Somethings wrong</span>');

    if( empty( $comment ) )
        die('<span class="error">Please enter your comment</span>');

    if( empty( $name ) )
        die('<span class="error">please enter your name</span>');

    if( !empty( $email ) && !filter_var($email, FILTER_VALIDATE_EMAIL) )
        die('<span class="error">the entered email doesnt look like an email address</span>');

    if( empty( $name ) && empty( $comment ) )
        die('<span class="error">please fill the form</span>');

    /* send testimonial */
    $testimonial = array(
        'post_title'  => $name,
        'post_status' => 'pending',
        'post_type'   => 'testimonials',
        'post_author' => 1
    );

    $post_id = wp_insert_post( $testimonial );

    update_field( 'job', $job, $post_id );
    update_field( 'email', $email, $post_id );
    update_field( 'comment', $comment, $post_id );

    // form is valid
    if( empty( $message ) )
        $message = '<span class="success">Your comment submitted! thank you.</span>';

    echo $message;
    wp_die();
}

edited :
I’ve run a test here ( sorry the page is in farsi ) : http://test.dornaweb.ir/ , there is a form in the middle of the page that when you click it , it shows you a var_dump() of $_SESSION['captcha'] , as you can see , the code shown in the image is different is from the code in var_dump data , it’s like when the form submits the $_SESSION is one step ahead or something like that , the weird thing is when i use the exact same theme on localhost nothing goes wrong!!