How to clear captcha image background color?

I have issued to clear background image or color in captcha, because my company client need to see clear background image in captcha for the user. Anyone can help me how to make captcha image background only show the white color. Below is my coding:

phptextClass.php (In this file, the background color will set random choose, how to remove random function to change fix to white color background)

   <?php

  class phptextClass
  { 
        public function 
      phptext($text,$textColor,$backgroundColor='',$fontSize,$imgWidth,$imgHeight,$dir,$fileName)
    {
    /* settings */
    $font = './calibri.ttf';/*define font*/
    $textColor=$this->hexToRGB($textColor); 

    $im = imagecreatetruecolor($imgWidth, $imgHeight);  
    $textColor = imagecolorallocate($im, $textColor['r'],$textColor['g'],$textColor['b']);  

    if($backgroundColor==''){/*select random color*/
        $colorCode=array('#56aad8', '#61c4a8', '#d3ab92');
        $backgroundColor = $this->hexToRGB($colorCode[rand(0, count($colorCode)-1)]);
        $backgroundColor = imagecolorallocate($im, 
     $backgroundColor['r'],$backgroundColor['g'],$backgroundColor['b']);
    }else{/*select background color as provided*/
        $backgroundColor = $this->hexToRGB($backgroundColor);
        $backgroundColor = imagecolorallocate($im, 
         $backgroundColor['r'],$backgroundColor['g'],$backgroundColor['b']);
    }

    imagefill($im,0,0,$backgroundColor);    
    list($x, $y) = $this->ImageTTFCenter($im, $text, $font, $fontSize); 
    imagettftext($im, $fontSize, 0, $x, $y, $textColor, $font, $text);
    if(imagejpeg($im,$dir.$fileName,90)){/*save image as JPG*/
        return json_encode(array('status'=>TRUE,'image'=>$dir.$fileName));
    imagedestroy($im);  
    }
}   
 public function phpcaptcha($textColor,$backgroundColor,$imgWidth,$imgHeight,$noiceLines=0,$noiceDots=0,$noiceColor='#162453')
{   
    /* Settings */
    $text=$this->random();
    $font = './font/monofont.ttf';/* font */
    $textColor=$this->hexToRGB($textColor); 
    $fontSize = $imgHeight * 0.75;

    $im = imagecreatetruecolor($imgWidth, $imgHeight);  
    $textColor = imagecolorallocate($im, $textColor['r'],$textColor['g'],$textColor['b']);          

    $backgroundColor = $this->hexToRGB($backgroundColor);
    $backgroundColor = imagecolorallocate($im, 
    $backgroundColor['r'],$backgroundColor['g'],$backgroundColor['b']);

    /* generating lines randomly in background of image */
    if($noiceLines>0){
    $noiceColor=$this->hexToRGB($noiceColor);   
    $noiceColor = imagecolorallocate($im, $noiceColor['r'],$noiceColor['g'],$noiceColor['b']);
    for( $i=0; $i<$noiceLines; $i++ ) {             
        imageline($im, mt_rand(0,$imgWidth), mt_rand(0,$imgHeight),
        mt_rand(0,$imgWidth), mt_rand(0,$imgHeight), $noiceColor);
    }}              

    if($noiceDots>0){/* generating the dots randomly in background */
    for( $i=0; $i<$noiceDots; $i++ ) {
        imagefilledellipse($im, mt_rand(0,$imgWidth),
        mt_rand(0,$imgHeight), 3, 3, $textColor);
    }}      

    imagefill($im,0,0,$backgroundColor);    
    list($x, $y) = $this->ImageTTFCenter($im, $text, $font, $fontSize); 
    imagettftext($im, $fontSize, 0, $x, $y, $textColor, $font, $text);      

    imagejpeg($im,NULL,90);/* Showing image */
    header('Content-Type: image/jpeg');/* defining the image type to be shown in browser widow */
    imagedestroy($im);/* Destroying image instance */
    if(isset($_SESSION)){
        $_SESSION['captcha_code'] = $text;/* set random text in session for captcha validation*/
    }
}

   /*for random string*/
  protected function random($characters=6,$letters = '23456789bcdfghjkmnpqrstvwxyz'){
    $str='';
    for ($i=0; $i<$characters; $i++) { 
        $str .= substr($letters, mt_rand(0, strlen($letters)-1), 1);
    }
    return $str;
    }   

 /*function to convert hex value to rgb array*/
protected function hexToRGB($colour)
{
        if ( $colour[0] == '#' ) {
                $colour = substr( $colour, 1 );
        }
        if ( strlen( $colour ) == 6 ) {
                list( $r, $g, $b ) = array( $colour[0] . $colour[1], $colour[2] . $colour[3], $colour[4] . $colour[5] );
        } elseif ( strlen( $colour ) == 3 ) {
                list( $r, $g, $b ) = array( $colour[0] . $colour[0], $colour[1] . $colour[1], $colour[2] . $colour[2] );
        } else {
                return false;
        }
        $r = hexdec( $r );
        $g = hexdec( $g );
        $b = hexdec( $b );
        return array( 'r' => $r, 'g' => $g, 'b' => $b );
}       

/*function to get center position on image*/
protected function ImageTTFCenter($image, $text, $font, $size, $angle = 8) 
{
    $xi = imagesx($image);
    $yi = imagesy($image);
    $box = imagettfbbox($size, $angle, $font, $text);
    $xr = abs(max($box[2], $box[4]));
    $yr = abs(max($box[5], $box[7]));
    $x = intval(($xi - $xr) / 2);
    $y = intval(($yi + $yr) / 2);
    return array($x, $y);   
}
  }
  ?>

captcha.php

    <?php
    session_start();
    include("./phptextClass.php");  

   /*create class object*/
   $phptextObj = new phptextClass();    
    /*phptext function to genrate image with text*/
     $phptextObj->phpcaptcha('#162453','#fff',120,40,10,25);    
   ?>

Style.css

      body {
      font-family: Arial, Helvetica, sans-serif;
        font-size: 12px;
      color: #333;
       background-color: #FFF;
       }
     a {
    -moz-user-select: none;
  color: #1155CC !important;
    font-family: "verdana", "Helvetica Neue", Helvetica, Arial, sans-serif;
     text-decoration: none;
 }
   a:hover {
text-decoration: underline;
   }
 #frame1, #frame0 {
background-color: #F7F7F7;
margin: 30px auto auto;
padding: 10px;
width: 750px;
border: 1px solid #EEE;
}
#fade {
background: none repeat scroll 0 0 #D3DCE3;
display: none;
height: 100%;
left: 0;
opacity: 0.4;
position: fixed;
top: 0;
width: 100%;
z-index: 99;
}
#centerBox {
background-color: #FFFFFF;
border: 5px solid #FFFFFF;
border-radius: 2px 2px 2px 2px;
box-shadow: 0 1px 3px rgba(34, 25, 25, 0.4);
display: none;
max-height: 480px;
overflow: auto;
visibility: hidden;
width: 710px;
z-index: 100;
  }
 .box1 {
background: none repeat scroll 0 0 #F3F7FD;
border: 1px solid #D3E1F9;
font-size: 12px;
margin-top: 5px;
padding: 4px;
}
.button1 {
background-color: #FFFFFF;
background-image: -moz-linear-gradient(center bottom, #EDEDED 30%, #FFFFFF 83%);
border-color: #999999;
border-radius: 2px 2px 2px 2px;
border-style: solid;
border-width: 1px;
box-shadow: 0 1px 0 rgba(0, 0, 0, 0.1);
color: #333333;
cursor: pointer;
display: inline-block;
font-family: Arial, Helvetica, sans-serif;
font-size: 11px;
font-weight: 700;
height: 25px;
line-height: 24px;
margin-right: 2px;
min-width: 40px;
padding: 0 16px;
text-align: center;
text-decoration: none;
-webkit-user-select: none;  /* Chrome all / Safari all */
-moz-user-select: none;     /* Firefox all */
-ms-user-select: none;      /* IE 10+ */
}
   .button1:hover {
   text-decoration: underline;
  }
 .button1:active, .a:active {
    position: relative;
   top: 1px;
  }
  .table {
  font-family: verdana, Helvetica, sans-serif;
  font-size: 12px;
  color: #333;
 background-color: #E4E4E4;
  }
 .table td {
  background-color: #F8F8F8;
  }

Below is my output (I know can press refresh to choose the clear image, but the client want direct change to white background color inside the image):

The client want the output same like below the picture, he want very clear can see the validation code inside the box:

Hope someone can guide me how to change background color to fix white color. I have tried many ways to solve it, but didn't work.