The final output should be like image(HELLO WORLD):

Here is what i am do
This is not the exact thing what you may be looking for, but i believe it will help you
$im = imagecreate(400,400);
$txtcol = imagecolorallocate($im, 0xFF, 0xFF, 0x00);
$size = 20;
$txt = 'DUMMY TEXT';
$font = 'Capriola-Regular.ttf';
/*two points for base line*/
$x1 = 70; $x2 = 170;
$y1 = 140; $y2 = 240;
/*bof finding line angle*/
$delta_x = $x2-$x1;
$delta_y = $y2-$y1;
$texangle = (rad2deg(atan2($delta_y,$delta_x)) * 180 / M_PI)-360;
/*eof finding the line angle*/
imageline($im,$x1,$y1,$x2,$y2,'0xDD'); //Drawing line
imagettftext($im, $size, -45, $x1, $y1-10, '0xDD', $font, $txt); // Drawing text over line at line's angle
header('Content-Type: image/png');
imagepng($im);
imagedestroy($im);
values are hard coded. Here the angle is given as -45 and for that angle if you want your text to appear above line you have to reduce the value from initial Y position of your line. And you will have to write code to calculate the angle with line's x1,x2y1,y2