problem in aligning text on an image

I am using a function that write text on image, the problem is that it sometimes do not center align text and sometimes some characters missed. I use the following logic in the function, i set no of pixels for one character and multiply it with the total no of characters and divide it with the width in pixels of an image, but it is not working accurate at sometime because, i set the character pixels myself if know some way how to get pixels of a character because some characters takes more pixels and some less i.e caps characters. the code is as follows plz suggest a solution

function writeOnImage($filePath, $title, $author, $subtitle, $title_R, $title_G, $title_B, $subtitle_R, $subtitle_G, $subtitle_B, $title_fontsize, $subtitle_fontsize, $author_R, $author_G, $title_B, $author_fontsize,$finalImage)
{
//$existingImage = substr($existingImage, 3);
$im = imagecreatetruecolor(90, 30);
$title_fontColor = imagecolorallocate($im, $title_R, $title_G, $title_?;
$subtitle_fontColor = imagecolorallocate($im, $subtitle_R, $subtitle_G, $subtitle_?;
$author_fontColor = imagecolorallocate($im, $author_R, $author_G, $author_?;
// Create some colors
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, 399, 29, $white);
// Replace path by your own font path
$font = ‘arial.ttf’;
$im_bg = @imagecreatefromjpeg($filePath);
//imagejpeg($im,”New.jpeg”, 100);
list($width, $height) = getimagesize($filePath);
$pixels_for_one_char = 16;
$available_pixels = (int) $width ;
//echo “ONE: “.pixels_for_one_char;
//echo ” TITLE::”. $title;
$title_length = strlen($title);
$pixels_for_whole_title = $title_length*$pixels_for_one_char;
if($pixels_for_whole_title > $available_pixels)
{
$no_of_lines = ceil($pixels_for_whole_title/$available_pixels);
}
else
{
$no_of_lines =1;
}
//echo “LINES”.$no_of_lines ;
$y = 70;
$from=0;
$chars_in_one_line = round($available_pixels/$pixels_for_one_char);
$limit = $chars_in_one_line;
$white = imagecolorallocate($im_bg, 255, 255, 255);
$str = wordwrap($title,$chars_in_one_line,”#”);
$title_in_lines = explode(“#”,$str);

for($i=0; $i<count($title_in_lines); $i++)
{
//echo “loop”.$title_in_lines[$i];
//$text_to_write = substr($title,$from,$limit);
$total_pixels_for_title = strlen($title_in_lines[$i])*$pixels_for_one_char;
$xPosition = ( $available_pixels – $total_pixels_for_title)/2 ;
//$from = ($i+1)*$chars_in_one_line;
//$limit = $from + $chars_in_one_line;
imagettftext($im_bg, $title_fontsize, 0, $xPosition, $y, $title_fontColor, $font, $title_in_lines[$i]);
imagettftext($im_bg, $title_fontsize, 0, ($xPosition), $y-1, $title_fontColor, $font, $title_in_lines[$i]);
$y= $y+50;

}
////////////// write subtitle//////////////////////
$y = $y – 18;
$pixels_for_one_char = 8;
$available_pixels = (int) $width ;
//echo “ONE: “.pixels_for_one_char;
//echo ” TITLE::”. $title;
$title_length = strlen($subtitle);
$pixels_for_whole_title = $title_length*$pixels_for_one_char;
if($pixels_for_whole_title > $available_pixels)
{
$no_of_lines = ceil($pixels_for_whole_title/$available_pixels);
}
else
{
$no_of_lines =1;
}
//echo “LINES”.$no_of_lines ;

$chars_in_one_line = round($available_pixels/$pixels_for_one_char);
$limit = $chars_in_one_line;
$white = imagecolorallocate($im_bg, 255, 255, 255);
$str = wordwrap($subtitle,$chars_in_one_line,”#”);
$title_in_lines = explode(“#”,$str);

for($i=0; $i<count($title_in_lines); $i++)
{
//echo “loop”.$title_in_lines[$i];
//$text_to_write = substr($title,$from,$limit);
$total_pixels_for_title = strlen($title_in_lines[$i])*$pixels_for_one_char;
$xPosition = ( $available_pixels – $total_pixels_for_title)/2 ;
//$from = ($i+1)*$chars_in_one_line;
//$limit = $from + $chars_in_one_line;
imagettftext($im_bg, $subtitle_fontsize, 0, $xPosition, $y, $subtitle_fontColor, $font, $title_in_lines[$i]);
imagettftext($im_bg, $subtitle_fontsize, 0, ($xPosition), $y-1, $subtitle_fontColor, $font, $title_in_lines[$i]);
$y= $y+50;

}

///////////////////////// write author/////////////
//$xPosition = (int) $width / 4;
$pixels_for_one_char = 16;
$available_pixels = (int) $width ;
$author_length = strlen($author);
$total_pixels_for_author = $author_length*$pixels_for_one_char;
$xPosition = ( $available_pixels – $total_pixels_for_author)/2 ;

$yPosition = $height – 10;
$path_parts = pathinfo($filePath);
//echo “<br> Extension”.$path_parts[‘extension’];

imagettftext($im_bg, $author_fontsize, 0, $xPosition, $yPosition-89, $author_fontColor, $font, $author);
imagettftext($im_bg, $author_fontsize, 0, ($xPosition), ($yPosition – 90), $author_fontColor, $font, $author);

//header(“Content-type: image/jpeg”);
imagejpeg($im_bg, $finalImage, 100);
}// end write

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