How The PHP Stack Frame Works

Introduction

The PHP stack frame isn't as straight-forward as you might think. This caught us out when building FilterBacktrace.

What's In A Stack Frame

Here's a stack dump generated by calling debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS):

array(11) {
  [0] =>
  array(5) {
    'file' =>
    string(98) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/tests/V1/Exceptions/BadRequirementDataTest.php"
    'line' =>
    int(142)
    'function' =>
    string(22) "newFromRequirementData"
    'class' =>
    string(57) "GanbaroDigital\Defensive\V1\Exceptions\BadRequirementData"
    'type' =>
    string(2) "::"
  }
  [1] =>
  array(3) {
    'function' =>
    string(35) "testCanCreateFromBadRequirementData"
    'class' =>
    string(65) "GanbaroDigitalTest\Defensive\V1\Exceptions\BadRequirementDataTest"
    'type' =>
    string(2) "->"
  }
  [2] =>
  array(5) {
    'file' =>
    string(101) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/Framework/TestCase.php"
    'line' =>
    int(909)
    'function' =>
    string(10) "invokeArgs"
    'class' =>
    string(16) "ReflectionMethod"
    'type' =>
    string(2) "->"
  }
  [3] =>
  array(5) {
    'file' =>
    string(101) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/Framework/TestCase.php"
    'line' =>
    int(768)
    'function' =>
    string(7) "runTest"
    'class' =>
    string(26) "PHPUnit_Framework_TestCase"
    'type' =>
    string(2) "->"
  }
  [4] =>
  array(5) {
    'file' =>
    string(103) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/Framework/TestResult.php"
    'line' =>
    int(612)
    'function' =>
    string(7) "runBare"
    'class' =>
    string(26) "PHPUnit_Framework_TestCase"
    'type' =>
    string(2) "->"
  }
  [5] =>
  array(5) {
    'file' =>
    string(101) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/Framework/TestCase.php"
    'line' =>
    int(724)
    'function' =>
    string(3) "run"
    'class' =>
    string(28) "PHPUnit_Framework_TestResult"
    'type' =>
    string(2) "->"
  }
  [6] =>
  array(5) {
    'file' =>
    string(102) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/Framework/TestSuite.php"
    'line' =>
    int(747)
    'function' =>
    string(3) "run"
    'class' =>
    string(26) "PHPUnit_Framework_TestCase"
    'type' =>
    string(2) "->"
  }
  [7] =>
  array(5) {
    'file' =>
    string(100) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/TextUI/TestRunner.php"
    'line' =>
    int(440)
    'function' =>
    string(3) "run"
    'class' =>
    string(27) "PHPUnit_Framework_TestSuite"
    'type' =>
    string(2) "->"
  }
  [8] =>
  array(5) {
    'file' =>
    string(97) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/TextUI/Command.php"
    'line' =>
    int(149)
    'function' =>
    string(5) "doRun"
    'class' =>
    string(25) "PHPUnit_TextUI_TestRunner"
    'type' =>
    string(2) "->"
  }
  [9] =>
  array(5) {
    'file' =>
    string(97) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/TextUI/Command.php"
    'line' =>
    int(100)
    'function' =>
    string(3) "run"
    'class' =>
    string(22) "PHPUnit_TextUI_Command"
    'type' =>
    string(2) "->"
  }
  [10] =>
  array(5) {
    'file' =>
    string(82) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/phpunit"
    'line' =>
    int(47)
    'function' =>
    string(4) "main"
    'class' =>
    string(22) "PHPUnit_TextUI_Command"
    'type' =>
    string(2) "::"
  }
}

Each stack frame can contain any of these details:

At first glance, that looks very straight-forward. Let's isolate a single stack frame:

Hang on a moment. According to that, line 440 of TestRunner.php is part of the run() method of PHPUnit_Framework_TestSuite. Surely that's a typo?

No, it isn't. Here's the stack frame that I built that table from:

[7] =>
array(5) {
  'file' =>
  string(100) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/TextUI/TestRunner.php"
  'line' =>
  int(440)
  'function' =>
  string(3) "run"
  'class' =>
  string(27) "PHPUnit_Framework_TestSuite"
  'type' =>
  string(2) "->"
}

What's going on?

Stack Frames Contain Staggered Data

In any PHP stack frame, file and line correctly refer to the code that is executing. class, function and type (if present) refer to the code that file and line is calling - and not the code in file and line line as you might be expecting.

You can clearly see that in the following two stack frames:

[7] =>
array(5) {
  'file' =>
  string(100) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/TextUI/TestRunner.php"
  'line' =>
  int(440)
  'function' =>
  string(3) "run"
  'class' =>
  string(27) "PHPUnit_Framework_TestSuite"
  'type' =>
  string(2) "->"
}
[8] =>
array(5) {
  'file' =>
  string(97) "/Users/stuart/Devel/ganbarodigital/php-mv-defensive/vendor/phpunit/phpunit/src/TextUI/Command.php"
  'line' =>
  int(149)
  'function' =>
  string(5) "doRun"
  'class' =>
  string(25) "PHPUnit_TextUI_TestRunner"
  'type' =>
  string(2) "->"
}

How To Work Out Actual Caller Details

To work out a full set of caller details, do the following:

1. take the class, function and type values from a stack frame
2. take the file and line values from the preceeding stack frame

In our example above, if you combine class, function and type from stack frame 8 with file and line from stack frame 7, you get this:

... which looks far more credible :)

Automatically Handled For You

Our FilterBacktrace class automatically handles this for you. All of our other classes in the TraceInspectors namespace use FilterBacktrace to understand the PHP stack.

As long as you use our classes, you won't have to worry about this PHP behaviour in your own code.