<?php

shell_exec("convert input.png
          -gravity Southwest   -background '#f48fb0'  -splice 0x44
          -pointsize 30 -fill white -annotate +10+4 'This is a long caption texts that I need to place in the image caption... blah blah blah blah... This is a long caption texts that I need to place in the image caption... blah blah blah blah... '   output.png");

?>

wd=`convert logo: -format "%w" info:`

convert logo:
( -size ${wd}x -background '#f48fb0' -gravity center -fill white
label:'This is a long caption texts that I need to place in the image caption... blah blah blah blah... This is a long caption texts that I need to place in the image caption... blah blah blah blah... ' )
-append output2.png

convert logo:
( -size ${wd}x -background '#f48fb0' -gravity center -fill white -pointsize 30
caption:'This is a long caption texts that I need to place in the image caption... blah blah blah blah... This is a long caption texts that I need to place in the image caption... blah blah blah blah... ' )
-append output3.png

<?php
$wd = exec("convert input.png -format '%w' info:");
exec("convert logo: ( -size ${wd}x -background '#f48fb0' -gravity center -fill white label:'This is a long caption texts that I need to place in the image caption... blah blah blah blah... This is a long caption texts that I need to place in the image caption... blah blah blah blah... ' ) -append output.png");
?>

<?php
exec("wd=`convert input.png -format '%w' info:`
convert input.png ( -size ${wd}x -background '#f48fb0' -gravity center -fill white label:'This is a long caption texts that I need to place in the image caption... blah blah blah blah... This is a long caption texts that I need to place in the image caption... blah blah blah blah... ' ) -append output.png");
?>