Preprocess a PDF form to fill it with FPDF: This will give unique names to all fields, flip checkbox options (yes needs to be before no) and makes all fields readonly.

convert-fpdf-form.php

<?php

echo "\e[32mConvert the form in a PDF to be used by FPDF. This will rename all form fields.\e[39m\n";
$filename = readline("PDF Path: ");
$content = file_get_contents($filename);

// ==== TASK 1 ====
// Find all text input fields and give them unique names.
// Solves: PDF forms are sometimes organized in groups where a form field has children (“/Kids”). Children of different
// groups could have the same field names. FPDF can't handle that.
// Solution: Give every form field unnecessary if parent or child a unique name.
$textFieldMatches = [];
preg_match_all('/\/T\s\(.+\)/', $content, $textFieldMatches);

echo sprintf("\nFound \e[31m%d\e[39m form fields.\n", count($textFieldMatches[0]));

$countTextFields = 1;
foreach ($textFieldMatches[0] as $match) {
    $find = '~' . preg_quote($match) . '~';
    $replacement = sprintf('/T (Field%03d)', $countTextFields);
    $content = preg_replace($find, $replacement, $content, 1);
    $countTextFields++;
}

// ==== TASK 2 ====
// Find all checkbox fields and give them unique names.
// Solves: Checkbox fields may be unnamed if they are a child of a named form field. FPDF does not find those unnamed
// checkbox fields.
// Solution: Find the “/AS” tag identifying checkboxes and add a unique field name (“/T (SomeName)”).
$checkboxMatches = [];
preg_match_all('/\/AS\s\//', $content, $checkboxMatches);

echo sprintf("Found \e[31m%d\e[39m checkbox fields.\n", count($checkboxMatches[0]));

$countCheckboxes = 1;
foreach ($checkboxMatches[0] as $match) {
    $find = "~(?<!\/T\s\(Checkbox\d{3}\)\n)(" . preg_quote($match) . ")~";
    $replacement = sprintf("/T (Checkbox%03d)\n$1", $countCheckboxes);
    $content = preg_replace($find, $replacement, $content, 1);
    $countCheckboxes++;
}

// ==== TASK 3 ====
// Solves: My PDF files listed the checkboxes “no“ option before the checkboxes “yes” option. FPDF interprets it vice
// versa.
// Solution: Flip both options. The options are always listed below “/AP” and “/D”.
$countFlipOptions = 0;
$content = preg_replace('/(?<=\/AP\s\n<<\n\/D\s\n<<\n)(\/.+)\n(\/.+)/', "$2\n$1", $content, -1, $countFlipOptions);

// ==== TASK 4 ====
// Make all input fields which do not already have an option definition (/Ff) readonly.
$countReadonlyFields = 0;
$content = preg_replace('/([^<>]+\/T\s\([^\)]+\)\n)(?![^<>]+\/Ff[^<>]+[<>])/ms', "$1/Ff 1\n", $content, -1 ,$countReadonlyFields);

file_put_contents($filename, $content);

echo sprintf(
    "\nReplaced \e[31m%d\e[39m form fields and \e[31m%d\e[39m checkbox fields.\nFlipped \e[31m%d\e[39m options.\nMade \e[31m%d\e[39m fields readonly.\n",
    $countTextFields - 1,
    $countCheckboxes - 1,
    $countFlipOptions,
    $countReadonlyFields
);