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