<?php
$string = 'intKey=6, floatKey=12.34, simpleString=simple_value, booleanValue=true, quotedString="test\"value",singlQuoted=' . 'singlQuotedValue' . ',nullValue=null';
$mess   = explode(",", $string);
for ($i = 0; $i < count($mess); $i++) {
    $mess1 = explode("=", $mess[$i]);
    if (is_float($mess1[1])) {
        $mess1[1] = (double) $mess1[1];
    }
    if (is_string($mess1[1])) {
        $mess1[1] = (string) $mess1[1];
    }
    if (is_numeric($mess1[1])) {
        $mess1[1] = (int) $mess1[1];
    }
    $k[$mess1[0]] = $mess1[1];
}
var_dump($k);