How can I capture the result of var_dump to a string?

Try var_export

You may want to check out var_export — while it doesn't provide the same output as var_dump it does provide a second $return parameter which will cause it to return its output rather than print it:

$debug = var_export($my_var, true);

Why?

I prefer this one-liner to using ob_start and ob_get_clean(). I also find that the output is a little easier to read, since it's just PHP code.

The difference between var_dump and var_export is that var_export returns a "parsable string representation of a variable" while var_dump simply dumps information about a variable. What this means in practice is that var_export gives you valid PHP code (but may not give you quite as much information about the variable, especially if you're working with resources).

Demo:

$demo = array(
    "bool" => false,
    "int" => 1,
    "float" => 3.14,
    "string" => "hello world",
    "array" => array(),
    "object" => new stdClass(),
    "resource" => tmpfile(),
    "null" => null,
);

// var_export -- nice, one-liner
$debug_export = var_export($demo, true);

// var_dump
ob_start();
var_dump($demo);
$debug_dump = ob_get_clean();

// print_r -- included for completeness, though not recommended
$debug_printr = print_r($demo, true);

The difference in output:

var_export ($debug_export in above example):

 array (
  'bool' => false,
  'int' => 1,
  'float' => 3.1400000000000001,
  'string' => 'hello world',
  'array' => 
  array (
  ),
  'object' => 
  stdClass::__set_state(array(
  )),
  'resource' => NULL, // Note that this resource pointer is now NULL
  'null' => NULL,
)

var_dump ($debug_dump in above example):

 array(8) {
  ["bool"]=>
  bool(false)
  ["int"]=>
  int(1)
  ["float"]=>
  float(3.14)
  ["string"]=>
  string(11) "hello world"
  ["array"]=>
  array(0) {
  }
  ["object"]=>
  object(stdClass)#1 (0) {
  }
  ["resource"]=>
  resource(4) of type (stream)
  ["null"]=>
  NULL
}

print_r ($debug_printr in above example):

Array
(
    [bool] => 
    [int] => 1
    [float] => 3.14
    [string] => hello world
    [array] => Array
        (
        )

    [object] => stdClass Object
        (
        )

    [resource] => Resource id #4
    [null] => 
)

Caveat: var_export does not handle circular references

If you're trying to dump a variable with circular references, calling var_export will result in a PHP warning:

 $circular = array();
 $circular['self'] =& $circular;
 var_export($circular);

Results in:

 Warning: var_export does not handle circular references in example.php on line 3
 array (
   'self' => 
   array (
     'self' => NULL,
   ),
 )

Both var_dump and print_r, on the other hand, will output the string *RECURSION* when encountering circular references.

Use output buffering:

<?php
ob_start();
var_dump($someVar);
$result = ob_get_clean();
?>

You could also do this:

$dump = print_r($variable, true);

if you are using PHP>=7.0.0

function return_var_dump(...$args): string
{
    ob_start();
    try {
        var_dump(...$args);
        return ob_get_clean();
    } catch (\Throwable $ex) {
        // PHP8 ArgumentCountError for 0 arguments, probably..
        // in php<8 this was just a warning
        ob_end_clean();
        throw $ex;
    }
}

or if you are using PHP >=5.3.0:

function return_var_dump(){
    ob_start();
    call_user_func_array('var_dump', func_get_args());
    return ob_get_clean();
}

or if you are using PHP<5.3.0 (this function is actually compatible all the way back to PHP4)

function return_var_dump(){
    $args = func_get_args(); // For <5.3.0 support ...
    ob_start();
    call_user_func_array('var_dump', $args);
    return ob_get_clean();
}

(prior to 5.3.0 there was a bug with func_get_args if used directly as an argument for another function call, so you had to put it in a variable and use the variable, instead of using it directly as an argument..)

You may also try to use the serialize() function. Sometimes it is very useful for debugging purposes.