How to generate a pdf on post saving or updating - php

Question

I'm trying to create a PDF from post content, including some ACF fields. Currently I've got it working on a POST form and dynamically creating a PDF each time, but I'd like to make it generate the PDF on saving the post so I can then just have a download button which downloads the PDF generated from saving the post, but having difficulty doing so. Any help would be grand!
I've tried to create this as a function in functions.php hooking into the acf/save_post action at prioirty 15 so it's after the initial save.
function vehicle_details_pdf( $post_id ) {
require('wp-content/themes/murrayscott-nelson/fpdf/fpdf.php');
class PDF extends FPDF
{
function Header()
{
$vehicleRef = $_POST['ref'];
$this->Image('wp-content/themes/murrayscott-nelson/assets/logo.jpg',10,10,80);
$this->SetFont('Arial','B',15);
$this->Cell(80);
$this->Cell(85,10,'Vehicle Details - Ref:' . $vehicleRef ,0,0,'R');
$this->Ln(20);
}
function Footer()
{
$this->SetY(-15);
$this->SetFont('Arial','I',8);
$this->Cell(0,10,'Page '.$this->PageNo().'/{nb}',0,0,'C');
}
$vehicleName = get_the_title();
$vehicleBodyStyle = get_field('body_type', $post_id);
$vehiclePrice = get_field('price', $post_id);
$vehicleEngCap = get_field('engine_capacity', $post_id);
$vehicleMileage = get_field('mileage', $post_id);
$vehicleSteering = get_field('steering', $post_id);
$vehicleDescription = get_field('description', $post_id);
$vehicleRef = get_field('reg_ref', $post_id);
$images = get_field('vehicle_images', $post_id);
$image = $images[0];
$image = $image['url'];
$vehicleImage = preg_replace("/^https:/i", "http:", $image);
$pdf = new PDF();
$pdf->AliasNbPages();
$pdf->AddPage();
$pdf->SetFont('Arial','B',17);
$pdf->Image( $vehicleImage, $pdf->GetX(), $pdf->GetY(), 190);
$pdf->Ln(130);
$pdf->Cell(0,10,$vehicleName);
$pdf->Ln(10);
$pdf->SetFont('Arial','',12);
$pdf->Cell(0,7,'Body Style: ' . $vehicleBodyStyle . ' | Steering: ' . $vehicleSteering . ' | Engine Capacity: ' . $vehicleEngineCap . 'cc | Mileage: ' . $vehicleMileage . ' miles' );
$pdf->Ln(7);
$pdf->Write (6, $vehicleDescription);
$pdf->SetFont('Arial','B',17);
$pdf->Ln(0);
$pdf->Cell( 0, 10, iconv("UTF-8", "ISO-8859-1", "£") . $vehiclePrice, 0);
$pdf->Ln(10);
$pdf->setFillColor(230,230,230);
$pdf->SetFont('Arial','B',15);
$pdf->MultiCell(0,7,"If you'd like to arrange a viewing please call 01723 361 227",0,'C',1);
$pdf->MultiCell(0,7,"quoting reference: " . $vehicleRef ,0,'C',1);
$pdf->setFillColor(38,54,29);
$pdf->setTextColor(255,255,255);
$pdf->Ln(5);
$pdf->MultiCell(0,7,"Finance Available - Ask for details" ,0,'C',1);
$pdf->setTextColor(0,0,0);
$filepath= wp_get_upload_dir() . '/vehicledetails/' ;
$pdf->Output('F', $filepath . $vehicleRef . '.pdf');
}
add_action('acf/save_post', 'vehicle_details_pdf', 15);
I expect the code to create a pdf file in the wp-content/uploads/vehicledetails directory
error messages:
[01-Sep-2019 10:49:10 UTC] PHP Warning: Invalid argument supplied for
foreach() in /app/public/wp-includes/class-wp-post-type.php on line
597 [01-Sep-2019 10:49:10 UTC] PHP Warning: Invalid argument supplied
for foreach() in /app/public/wp-includes/class-wp-post-type.php on
line 597 [01-Sep-2019 10:49:15 UTC] PHP Warning: Invalid argument
supplied for foreach() in
/app/public/wp-includes/class-wp-post-type.php on line 597
[01-Sep-2019 10:49:15 UTC] PHP Warning: Invalid argument supplied for
foreach() in /app/public/wp-includes/class-wp-post-type.php on line
597 [01-Sep-2019 10:49:17 UTC] PHP Warning: Invalid argument supplied
for foreach() in /app/public/wp-includes/class-wp-post-type.php on
line 597 [01-Sep-2019 10:49:17 UTC] PHP Warning: Invalid argument
supplied for foreach() in
/app/public/wp-includes/class-wp-post-type.php on line 597
[01-Sep-2019 10:49:17 UTC] PHP Warning: Invalid argument supplied for
foreach() in /app/public/wp-includes/class-wp-post-type.php on line
597 [01-Sep-2019 10:49:18 UTC] PHP Warning: Invalid argument supplied
for foreach() in /app/public/wp-includes/class-wp-post-type.php on
line 597 [01-Sep-2019 10:49:19 UTC] PHP Warning: Invalid argument
supplied for foreach() in
/app/public/wp-includes/class-wp-post-type.php on line 597
[01-Sep-2019 10:49:19 UTC] PHP Warning: Invalid argument supplied for
foreach() in /app/public/wp-includes/class-wp-post-type.php on line
597 [01-Sep-2019 10:49:20 UTC] PHP Warning: Invalid argument supplied
for foreach() in /app/public/wp-includes/class-wp-post-type.php on
line 597 [01-Sep-2019 10:49:20 UTC] PHP Warning: Invalid argument
supplied for foreach() in
/app/public/wp-includes/class-wp-post-type.php on line 597
[01-Sep-2019 10:49:21 UTC] PHP Warning:
require(wp-content/themes/murrayscott-nelson/fpdf/fpdf.php): failed to
open stream: No such file or directory in
/app/public/wp-content/themes/murrayscott-nelson/functions.php on line
81 [01-Sep-2019 10:49:21 UTC] PHP Warning:
require(wp-content/themes/murrayscott-nelson/fpdf/fpdf.php): failed to
open stream: No such file or directory in
/app/public/wp-content/themes/murrayscott-nelson/functions.php on line
81 [01-Sep-2019 10:49:21 UTC] PHP Fatal error: require(): Failed
opening required 'wp-content/themes/murrayscott-nelson/fpdf/fpdf.php'
(include_path='.:/usr/share/php:/www/wp-content/pear') in
/app/public/wp-content/themes/murrayscott-nelson/functions.php on line
81 [01-Sep-2019 10:49:21 UTC] PHP Fatal error: require(): Failed
opening required 'wp-content/themes/murrayscott-nelson/fpdf/fpdf.php'
(include_path='.:/usr/share/php:/www/wp-content/pear') in
/app/public/wp-content/themes/murrayscott-nelson/functions.php on line
81 [01-Sep-2019 10:50:16 UTC] PHP Warning: Invalid argument supplied
for foreach() in /app/public/wp-includes/class-wp-post-type.php on
line 597 [01-Sep-2019 10:52:17 UTC] PHP Warning: Invalid argument
supplied for foreach() in
/app/public/wp-includes/class-wp-post-type.php on line 597
[01-Sep-2019 10:54:18 UTC] PHP Warning: Invalid argument supplied for
foreach() in /app/public/wp-includes/class-wp-post-type.php on line
597 [01-Sep-2019 10:56:18 UTC] PHP Parse error: syntax error,
unexpected '$vehicleName' (T_VARIABLE), expecting function
(T_FUNCTION) or const (T_CONST) in
/app/public/wp-content/themes/murrayscott-nelson/functions.php on line
114 [01-Sep-2019 10:58:19 UTC] PHP Parse error: syntax error,
unexpected '$vehicleName' (T_VARIABLE), expecting function
(T_FUNCTION) or const (T_CONST) in
/app/public/wp-content/themes/murrayscott-nelson/functions.php on line
114

Answer 1

Your upload directory is not set correctly.
Try this:
$uploads = wp_upload_dir();
$pdf->Output('F', $uploads['basedir'] . '/vehicledetails/' . $vehicleRef . '.pdf');