PHP Fails with loose string comparison?

Question

This is my first post, I hope I'm doing everything OK asking my question: I want to create a function which detects whether a string is valid JSON or not. I already read good topic here (Fastest way to check if a string is JSON in PHP?) but I wanted to add the check functionality for the first character as this function will get lot of non-JSON input. Yet it keeps failing, I don't know why...

Here's the function I wrote so far:

function isJson($string){
    $string = trim($string);
    vardump($string);  //for debugging
    if($string[0]!='{'||$string[0]!='['){ //check for first char
      echo("{$string[0]}!={ OR {$string[0]}!=["); //this actually prints {!={ OR {!=[ every time it comes across valid JSON...
      return false;
    }
    if(json_decode($string)==true)return true;
    else return false;
    }

Could you help me with this please, as it is starting to drive me insane? Thank you in advance!

what does you vardump look like? ( and it should be var_dump($string)) — Jim
– Jim, Commented May 30, 2013 at 15:30
Not sure if you gain any speed by checking the first character yourself. I guess the "json_decode" is clever enough to stop parsing as soon as it detects that a json string is not correctly formatted. Wild guess , but you should benchmark it. — Frédéric Clausset
– Frédéric Clausset, Commented May 30, 2013 at 15:32
@Jim: My vardump looks like this: string(65) "[{"os":"iOS","hardware":"iPad"},{"os":"iOS","hardware":"iPhone"}]" vardump is my function which only wraps var_dump() in <pre> tags. — Mark Johann
– Mark Johann, Commented May 30, 2013 at 15:37
@FrédéricClausset: Thanks, once this gets sorted out I'll benchmark it and post the results here! — Mark Johann
– Mark Johann, Commented May 30, 2013 at 15:39
woah we missed the obvious ! of course this is always true if($string[0]!='{'||$string[0]!='[') , you need an && — Frédéric Clausset
– Frédéric Clausset, Commented May 30, 2013 at 15:45

mpratt · Accepted Answer · 2013-05-30 15:35:39Z

2

I would be using json_decode only.

function isJson($string)
{
    return (json_decode($string) !== null);
}

But If you insist on checking the first chars, then I would use substr. And I believe your if statement should be using && instead of ||

if ($string[0] != '{' && $string[0] != '['){ .....

Or something like this

if (!in_array(substr($string, 0, 1), array('{', '[')))
    return false;

answered May 30, 2013 at 15:35

mpratt

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Comments

jimmy · Accepted Answer · 2013-05-30 15:34:47Z

0

json_encode returns false if the string is not valid Json.

function isValidJson($string) {
    if (json_encode($string) === false) {
        return false;
    }
    return true;
}

answered May 30, 2013 at 15:34

jimmy

11 bronze badge

1 Comment

Mark Johann Over a year ago

Thanks, I know that, but I think it'll be a performance improvement to instantly return false if the first character makes being valid json impossible. Will benchmark later.

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