<?php
$SQUARESIZE = 108;  // the size in pixels for your square images
$convertPath = "/usr/local/bin/convert";   // full path to ImageMagick's convert tool

$path = getcwd()."/";  // path to folder full of jpg files to convert

$n = 0;

if ($handle = opendir($path)) {
    while (false !== ($file = readdir($handle))) {
        if ($file != "." && $file != ".." && strpos($file,"jpg") !== false) {
            echo $file."\n";
            $originalname = $file;

            $filenamePart = $n++;

            $squarename = $filenamePart."_sq.jpg";
            $squareCommand = $convertPath." \"".$path.$originalname."\" -thumbnail x".($SQUARESIZE*2)." -resize '".($SQUARESIZE*2)."x&lt;' -resize 50% -gravity center -crop ".$SQUARESIZE."x".$SQUARESIZE."+0+0 +repage -format jpg -quality 91 \"".$path."sq/".$squarename."\"";
            system($squareCommand);
        }
    }
    closedir($handle);
}
?>

http://www.randomsequence.com/articles/making-square-thumbnails-with-imagemagick/

PHP Code to generate thumbnails of all images in current directory

PHP