GD script fails to process all images in array - php

Question

I've been building a gallery in php as a learning exercise and also because I'll need one to display my wedding photos in a couple of months.
I have written a function that processes the images. It looks to see what images are in the original folder and generates a thumbnail and preview for each. It's worked fine up until now as I've been testing with small batches of images.
Now I have 24 images in the original folder it only process the first 18. I don't get any errors displayed it just seems to stop.
Function
function processImgs($previewWidth, $thumbWidth, $thumbPath, $previewPath, $originalPath) {
$images = glob($originalPath."*");
foreach($images as $image){
$path_parts = pathinfo($image);
// setup filenames
$thumbFile = $thumbPath . "thumb_" . $path_parts['filename'] . ".jpg";
$previewFile = $previewPath . "preview_" . $path_parts['filename'] . ".jpg";
// calculate new images sizes
$size = getimagesize($image);
$originalWidth = $size['0'];
$originalHeight = $size['1'];
$previewHeight = $originalHeight / ($originalWidth / $previewWidth);
$thumbHeight = $originalHeight / ($originalWidth / $thumbWidth);
//create new images
$original = imagecreatefromjpeg($image);
$preview = imagecreatetruecolor($previewWidth, $previewHeight);
$thumb = imagecreatetruecolor($thumbWidth, $thumbHeight);
imagecopyresampled($preview, $original, 0, 0, 0, 0, $previewWidth, $previewHeight, $originalWidth, $originalHeight);
imagecopyresampled($thumb, $original, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $originalWidth, $originalHeight);
// save images to their new folders
imagejpeg($preview, $previewFile);
imagejpeg($thumb, $thumbFile);
imagedestroy($original);
imagedestroy($preview);
imagedestroy($thumb);
// give some feedback when the script runs
if (file_exists($thumbFile) && file_exists($previewFile)) {
echo ". " . $path_parts['filename'] . " processed successfully.<br>";
} else {
echo ". " . $path_parts['filename'] . " <strong>failed.</strong><br>";
}
}
}
I call the function like this
processImgs(1200, 400, "thumb/", "preview/", "original/");
I don't know why it doesn't work with more files. Is there a time limit that I could be hitting? It does take a while. I'm currently building this locally on MAMP.
UPDATE
Mike was correct, the function was timing out. His solution is much more suitable.
I have now created a script that does what he suggested and it has solved the problem. Thanks for the help. I've added the script below in case it's of use to others.
$newWidth = 400;
if (isset($_GET['img'])) {
$img = $_GET['img'];
$filename = $img . ".jpg";
if (file_exists($filename)) {
$fp = fopen($filename, 'rb');
header("Content-Type: image/jpeg");
header("Content-Length: " . filesize($filename));
fpassthru($fp);
exit;
} else {
$originalFile = "../original/" . $filename;
$original = imagecreatefromjpeg($originalFile);
$size = getimagesize($originalFile);
$newHeight = $size['1'] / ( $size['0'] / $newWidth);
$new = imagecreatetruecolor($newWidth, $newHeight);
imagecopyresampled($new, $original, 0, 0, 0, 0, $newWidth, $newHeight, $size['0'], $size['1']);
imagejpeg($new, $filename);
imagedestroy($original);
imagedestroy($new);
$fp = fopen($filename, 'rb');
header("Content-Type: image/jpeg");
header("Content-Length: " . filesize($filename));
fpassthru($fp);
}
}