json_decode() returns null issues

I've an issue with my JSON. It works returns correctly in PHP 5.3 (so I can't use json_last_error()), and it returns successfully when I copy string explicitly into json_decode (json_decode('{...}'). It only returns null in when I pass the result as a variable and I'm using php 5.2, which is what I need it for.

The output comes from JSON logging in PHPUnit:

[
    {
        "event": "suiteStart",
        "suite": "",
        "tests": 2
    },
    {
        "event": "suiteStart",
        "suite": "TagTestCase",
        "tests": 2
    },
    {
        "event": "test",
        "suite": "TagTestCase",
        "test": "TagTestCase::test_it",
        "status": "fail",
        "time": 0.00248718261719,
        "trace": [
            {
                "file": "\/UnitTest\/PHPUnit.php",
                "line": 98,
                "function": "run",
                "class": "PHPUnit_Framework_TestSuite",
                "type": "->",
                "args": [
                    {

                    }
                ]
            },
            {
                "file": "\/UnitTest\/PHPUnit.php",
                "line": 116,
                "function": "run",
                "class": "PHPUnit",
                "type": "->",
                "args": [

                ]
            },
            {
                "file": "\/UnitTest\/PHPUnit.php",
                "line": 212,
                "function": "__tostring",
                "class": "PHPUnit",
                "type": "->",
                "args": [

                ]
            }
        ],
        "message": "false assertionzzzzz.\nFailed asserting that <boolean:false> is true."
    },
    {
        "event": "test",
        "suite": "TagTestCase",
        "test": "TagTestCase::test_two",
        "status": "pass",
        "time": 0.00182914733887,
        "trace": [

        ],
        "message": ""
    }
]

EDIT: These are the paths, I've been exploring -