imageftbbox

imagettfbbox() returns an array with 8 elements representing four points making the bounding box of the text: 
0 lower left corner, X position 
1 lower left corner, Y position 
2 lower right corner, X position 
3 lower right corner, Y position 
4 upper right corner, X position 
5 upper right corner, Y position 
6 upper left corner, X position 
7 upper left corner, Y position 
The points are relative to the text regardless of the angle, so "upper left" means in the top left-hand corner seeing the text horizontally.

This function can be used to generate right-aligned text. Just work out how wide the text image is and position it accordingly. Example:
$i_width = 200;
$i_height = 40;
$string = "Hello World!";
$pointsize = 10;
$fontfile = "/usr/local/lib/ttf/Helve.ttf";
$im = imagecreate($i_width, $i_height);
$black = imagecolorallocate ($im, 0, 0, 0);
$white = imagecolorallocate ($im, 255, 255, 255);
$string_size = ImageFtBbox($pointsize, 0, $fontfile, $string, array("linespacing" => 1));
$s_width = $string_size[4];
$s_height = $string_size[5];
ImageFtText($im, $pointsize, 0, $i_width - $s_width - 1, 0 - $s_height, $white, $fontfile, $string, array("linespacing" => 1));
Header ("Content-type: image/png");
ImagePNG ($im);
ImageDestroy ($im);

Here is a handy example I used to center "dynamic text" onto an image. 
Ex. Say you want to center a clients IP Address onto a picture. 
$ip=$_SERVER['REMOTE_ADDR']; 
$details = imageftbbox($fontsize, 0, $font, $ip, array("linespacing" => 1));
$xcoord = ($imgwidth - $details[4]) / 2; // this will return the x coordinate centered to your specific image. Make sure you set $imgwidth to the width of the image you are using. 
imagettftext($image, $fontsize, 0, $xcoord, $ycoord, $fontcolor, $font, $ip);

ah... the problem between imageftbbox() and imagefttext() lies in the mirroring of the y-axes.
Below you see, for a font-size 16 the boudingboxes of "b", "p" and "bp":
< b: w=9 h=15
b(0,-1)
b(9,-1)
b(9,-16)
b(0,-16)
< p: w=9 h=16
p(0,4)
p(9,4)
p(9,-12)
p(0,-12)
< bp: w=20 h=20
bp(0,4)
bp(20,4)
bp(20,-16)
bp(0,-16)
If drawing "bp" using imagefttext() at y=0, the the top of "bp" indeed is at y=-16, and the bottom of "bp" at y=4. (Plus or minus a pixel here and there, because at y=0 there actually is a vissible pixel.)

//EXAMPLE - Center text
function newText($im, $size, $angle= 0, $x, $y, $color, $font, $text,$align = "left",$border=false,$width=0,$height=0){
 
 if($align == "center")
 {
 if ($border == true ){
 imagerectangle($im, $x, $y, $x +$width, $y + $height, $color);
 }
 $bbox = imageftbbox($size, 0, $font, $text);
 // Marcamos el ancho y alto
 $s_width = $bbox[4];
 $s_height = $bbox[5]; 
 
 $y = $y + ($height-$s_height)/2;
 $x = $x + ($width-$s_width)/2;
 }
 
 imagettftext($im, $size, $angle, $x, $y, $color, $font, $text);
}

For alignment i used this method:
if($align == "center" || $align == "right")
 {
 $verticaltxtspace = $backwidth - (2 * $posx); 
 $spacepositions = imagettfbbox($size, $angle, "fonts/verdanaz.ttf", " "); 
 $spacepx = $spacepositions[4] - $spacepositions[0]; 
 
 // Split text in lines
 $lines = split("[\r][\n]", $text); 
 for($count = 0; $count < count($lines); $count++)
 {
 $textpositions = imagettfbbox($size, $angle, "fonts/verdanaz.ttf", $lines[$count]); 
 $textpx = $textpositions[2] - $textpositions[0];
 
 if($align == "right")
 {
 $spaces = ($verticaltxtspace - $textpx) / $spacepx;
 }
 else if($align == "center")
 {
 $spaces = (($verticaltxtspace - $textpx)/2) / $spacepx;
 }
 
 // Add spaces
 $line = $lines[$count];
 for($i = 0; $i < $spaces; $i++)
 {
 $line = " " . $line;
 }
 $lines[$count] = $line;
 }
 
 // Create new text of lines
 $text = "";
 for($count = 0; $count < count($lines); $count++)
 {
 $text .= $lines[$count] . "\r\n";
 } 
 }
 
 
 // Draw the shadow text on de shadow
 imagettftext($background, $size, $angle, $posx, $posy, $textcolor, "fonts/verdanaz.ttf", $text);

i've found a work around for this situation
it seems that height is directly proportional to line spacing so you just have to apply the same factor to image height
for example :
$spacing = 0.7;
$params = array("linespacing" => $spacing);
$box = imageftbbox ($size, 0, $font, $text, $params);
$tw=$box[4]-$box[0]; //image width
$th=($box[1]-$box[5])*$spacing; //image height

ImageFTBBox returns a bounding box, not metrics, as some (most?) of the notes above seem to assume. The 8 values it returns specify the 4 corners of this bounding box. So to properly determine the width and height of a string you need to do:
$bbox = ImageFTBBox(...);
$width = abs($bbox[0]) + abs($bbox[2]); // distance from left to right
$height = abs($bbox[1]) + abs($bbox[5]); // distance from top to bottom