php: receive json from POST and save to file - php

Question

I want to receive a POST request from a JS client with a json body (i.e. this is not form data), and save the .gigs (javascript) array to a file, after checking the .password field. This is all my code (based on Receive JSON POST with PHP)
$json_params = file_get_contents("php://input");
if (strlen($json_params) > 0 && isValidJSON($json_params)){
/* json_decode(..., true) returns an 'array', not an 'object'
* Working combination: json_decode($json_params) WITH $inp->password
*/
$inp = json_decode($json_params);
} else {
echo "could not decode POST body";
return;
}
$password = $inp->password;
// echo $password;
if ($password == "****") {
$gigs = $inp['gigs'];
// WAS $res = file_put_contents('gigs.json', json_encode($gigs), TEXT_FILE);
$res = file_put_contents('gigs.json', json_encode($gigs));
if ($res > 0) {
echo "Success";
return;
} else {
if (!$res) {
http_response_code(500);
echo "file_put_contents error:".$res;
return;
} else {
http_response_code(500);
echo "Error: saved zero data!";
return;
}
}
}
else {
// http_response_code(403); // (2)
echo "Password invalid";
return;
}
What I find is that
if I comment out the if statement and uncomment echo $password; then the right password is there
if I uncomment line 2, which I want to do, then I get back a 500 and the error logs refer an Illegal string offset 'password' in line (1) above. Without that I get back a "Success" (all for the same password).
I don't understand what is happening, nor how to get 200, 403 and 500 error messages safely.

Answer 1

Note
$json_params = file_get_contents("php://input");
If your scripts are running upon regular HTTP requests, passing data like it comes from HTML form, them you should consider using $_POST for your content, not php://input. If you expect JSON in request body, then I'd be fine, yet I'd also check content type for application/json.
Next:
$inp = "I never got set";
if (strlen($json_params) > 0 && isValidJSON($json_params)){
$inp = json_decode($json_params, true);
}
$password = $inp->password;
$password = $inp['password'];
This is pretty broken. First, see json_decode() arguments (2nd) -> you are decoding to array (true), not object (false), so only $password = $inp['password']; will work in your case. Also the whole code will fail when your input data is invalid as in that case $np is rubbish string, not the array you try to read later on. Use null as default value and check for that prior further use.
Next:
$res = file_put_contents('gigs.json', json_encode($gigs), FILE_TEXT);
there's no FILE_TEXT option for file_put_contents(). Nor you'd need one.
Once you correct these you'd be fine. Also print_r() and var_dump() may be the functions you wish to get familiar with for your further debugging.
In general http://php.net/ -> lookup for functions you are about to use.