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5.9. Dumping Variable Contents as Strings

5.9.3. Discussion

The output of print_r( ) is more concise and easier to read. The output of var_dump( ), however, gives data types and lengths for each variable.

Since these functions recursively work their way through variables, if you have references within a variable pointing back to the variable itself, you can end up with an infinite loop. Both functions stop themselves from printing variable information forever, though. Once print_r( ) has seen a variable once, it prints *RECURSION* instead of printing information about the variable again and continues iterating through the rest of the information it has to print. When var_dump( ) sees a variable more than three times, it throws a fatal error and ends script execution. Consider the arrays $user_1 and $user_2, which reference each other through their friend elements:

$user_1 = array('name' => 'Max Bialystock',
                'username' => 'max');

$user_2 = array('name' => 'Leo Bloom',
                'username' => 'leo');

// Max and Leo are friends
$user_2['friend'] = &$user_1;
$user_1['friend'] = &$user_2;

// Max and Leo have jobs
$user_1['job'] = 'Swindler';
$user_2['job'] = 'Accountant';

The output of print_r($user_2) is:

Array
(
    [name] => Leo Bloom
    [username] => leo
    [friend] => Array
        (
            [name] => Max Bialystock
            [username] => max
            [friend] => Array
                (
                    [name] => Leo Bloom
                    [username] => leo
                    [friend] => Array
 *RECURSION*
                    [job] => Accountant
                )

            [job] => Swindler
        )

    [job] => Accountant
)

When print_r( ) sees the reference to $user_1 the second time, it prints *RECURSION* instead of descending into the array. It then continues on its way, printing the remaining elements of $user_1 and $user_2.

Confronted with recursion, var_dump( ) behaves differently:

array(4) {
  ["name"]=>
  string(9) "Leo Bloom"
  ["username"]=>
  string(3) "leo"
  ["friend"]=>
  &array(4) {
    ["name"]=>
    string(14) "Max Bialystock"
    ["username"]=>
    string(3) "max"
    ["friend"]=>
    &array(4) {
      ["name"]=>
      string(9) "Leo Bloom"
      ["username"]=>
      string(3) "leo"
      ["friend"]=>
      &array(4) {
        ["name"]=>
        string(14) "Max Bialystock"
        ["username"]=>
        string(3) "max"
        ["friend"]=>
        &array(4) {
          ["name"]=>
          string(9) "Leo Bloom"
          ["username"]=>
          string(3) "leo"
          ["friend"]=>
          &array(4) {
            ["name"]=>
            string(14) "Max Bialystock"
            ["username"]=>
            string(3) "max"
            ["friend"]=>
            &array(4) {
              ["name"]=>
              string(9) "Leo Bloom"
              ["username"]=>
              string(3) "leo"
              ["friend"]=>
              &array(4) {
<br />
<b>Fatal error</b>:  Nesting level too deep - recursive dependency? in 
<b>var-dump.php</b> on line <b>15</b><br />

It's not until the fourth appearance of the reference to $user_1 that var_dump( ) stops recursing. When it does, it throws a fatal error, and no more variable dumping (or script execution) occurs.

Even though print_r( ) and var_dump( ) print their results instead of returning them, you can capture the data without printing it using output buffering:

ob_start();
var_dump($user);
$dump = ob_get_contents();
ob_end_clean();

This puts the results of var_dump($user) in $dump.

5.9.4. See Also

Output buffering is discussed in Recipe 8.13; error handling with PEAR's DB module, shown in Recipe 10.9, uses output buffering with print_r( ) to save error messages; documentation on print_r( ) at http://www.php.net/print-r and var_dump( ) at http://www.php.net/var-dump .



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