Patching functions with when()

The first way Brain Monkey offers to monkey patch a function is Functions\when().

This function has to be used to set a behavior for functions.

when() and 5 related methods are used to define functions (if not defined yet) and:

  • make them return a specific value

  • make them return one of the received arguments

  • make them echo a specific value

  • make them echo one of the received arguments

  • make them behave just like another callback

For the sake of readability, in all the code samples below I'll assume that an use statement is in place:

use Brain\Monkey\Functions;

Don't forget to add it in your code as well, or use the fully qualified class name.

Also be sure to read the PHP Functions / Setup section that explain how setup Brain Monkey for usage in tests.


By using when() in combination with justReturn() you can make a (maybe) undefined function just return a given value:


echo a_undefined_function(); // echoes "Cool!"

Without passing a value to justReturn() the target function will return nothing (null).


This other when-related method is used to make the target function return one of the received arguments, by default the first.

Functions\when('give_me_the_first')->returnArg(); // is the same of ->returnArg(1)

echo give_me_the_first('A', 'B', 'C'); // echoes "A"
echo i_want_the_second('A', 'B', 'C'); // echoes "B"
echo and_the_third_for_me('A', 'B', 'C'); // echoes "C"

Note that if the target function does not receive the desired argument, returnArg() throws an exception:


// throws an exception because required 3rd argument, but received 2
echo needs_the_third('A', 'B');


Similar to justReturn(), it makes the mocked function echo some value instead of returning it.


a_undefined_function(); // echoes "Cool!"


Similar to returnArg(), it makes the mocked function echo some received argument instead of returning it.

Functions\when('echo_the_first')->echoArg(); // is the same of ->echoArg(1)

echo_the_first('A', 'B', 'C'); // echoes "A"
echo_the_second('A', 'B', 'C'); // echoes "B"


The last of the when-related methods allows to make a function behave just like another callback. The replacing function can be anything that can be run: a core function or a custom one, a class method, a closure...

Functions\when('duplicate')->alias(function($value) {
    return "Was ".$value.", now is ".($value * 2);


echo duplicate(1); // echoes "Was 1, now is 2"
echo bigger('was lower'); // echoes "WAS LOWER"

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