I don't know about performance or elegance of my solution, but it's what I'm using:

if (preg_match('/^\{\"/', $string)) {
    $aJson = json_decode($string, true);
    if (!is_null($aJson)) {
       ... do stuff here ...
    }
}

Since all my JSON encoded strings start with {" it suffices to test for this with a RegEx. I'm not at all fluent with RegEx, so there might be a better way to do this. Also: strpos() might be quicker.

Just trying to give in my tuppence worth.

I don't know about performance or elegance of my solution, but it's what I'm using:

if (preg_match('/^[\[\{]\"/', $string)) {
    $aJson = json_decode($string, true);
    if (!is_null($aJson)) {
       ... do stuff here ...
    }
}

Since all my JSON encoded strings start with {" it suffices to test for this with a RegEx. I'm not at all fluent with RegEx, so there might be a better way to do this. Also: strpos() might be quicker.

Just trying to give in my tuppence worth.

P.S. Just updated the RegEx string to /^[\[\{]\"/ to also find JSON array strings. So it now looks for either [" or {" at the beginning of the string.