Can I modify a variable within a string with another variable in PHP?

Question

$pet ='dog'; 
$action='My '. $pet . 'likes to run.'; 
//The part I would like to modify
$pet ='cat'; 
//Modify the $pet variable inside the $action variable 
//after it has been defined.
echo $action;

This will output: My dog likes to run

Can I make it output: My cat likes to run

I also tried:

$pet ='dog';  
$action=sprintf('My %s likes to run.', $pet);
$pet ='cat';
echo $action;

I do know that this could be achieved by creating a function with an argument like below. But I'm a php beginner and curious about the other methods.

function action($pet = "dog") {
    $p = 'My'. $pet . 'likes to run.';
    return $p;
}
$pet = 'cat';
echo action($pet);

The resulting string has effectively no memory of the parts it was built from. If you want one of the parts to be different, you'll have to rebuild it like you did with the function. — Shira
– Shira, Commented Oct 12, 2016 at 0:06
I was just writing a function for you to achieve this but I'm blind and didn't see yours. I would say that's the best way to do it. — Beans
– Beans, Commented Oct 12, 2016 at 0:28
This type of task is basically what functions are for. Return something based on a condition. Stick to the function. — worldofjr
– worldofjr, Commented Oct 12, 2016 at 0:29

Pieter van den Ham · Accepted Answer · 2016-10-12 10:41:07Z

Well, I know that you probably weren't looking for this, but it is late and I am bored.

Disclaimer: this is a bad idea.

class MyString
{
    private $format, $pet;

    public function __construct($format, &$pet)
    {
        $this->format = $format;
        $this->pet = &$pet;
    }

    public function __toString()
    {
        return sprintf($this->format, $this->pet);
    }
}

$myString = new MyString('This is my little %s, cool huh?', $pet);

$pet = 'cat';
echo $myString."\n";

$pet = 'dog';
echo $myString."\n";

$pet = 'goldfish';
echo $myString."\n";

Output:

This is my little cat, cool huh?
This is my little dog, cool huh?
This is my little goldfish, cool huh?

Demo: https://3v4l.org/XmUEZ

Basically this class stores a reference to the $pet variable in it's fields. As such, when the $pet variable is updated, the reference in the class is updated as well.

Another one for good measure:

function mysprintf($format, &$variable)
{
    return function() use ($format, &$variable) {
        return sprintf($format, $variable);
    };
}

$print = mysprintf('This is my little %s, cool huh?', $pet);

$pet = 'cat';
echo $print()."\n";

$pet = 'dog';
echo $print()."\n";

$pet = 'goldfish';
echo $print()."\n";

https://3v4l.org/KJTKj

(Ab)uses closures to save the reference. Probably even worse.

Why is this a bad idea, you ask?

Consider solely this statement:

$pet = 'goldfish';

This is a simple assignment. Most programmers assume that this statement has no side-effects. That means that this statement changes nothing in the execution flow besides creating a new variable.

Our MyString or mysprintf violate this assumption. What should be a simple assignment, now has side-effects. It violates a programmer's expectation in the worst way possible.

In real-world cases, this code is a nightmare for maintenance. Anyway, very clever use of variable reference with &$pet

A l w a y s S u n n y · Accepted Answer · 2016-10-12 00:55:55Z

2

You are trying to create a Dynamic String Variable, which is not possible in php for now. You can use the functional approach which you have implemented already :) .

function action($pet = "dog") {
    $p = 'My'. $pet . 'likes to run.';
    return $p;
}
$pet = 'cat';
echo action($pet);

if you use the below approach, unfortunately it will only covers printing part not when we need to reassign value of variable.

$pet ='dog';  
$action=sprintf('My %s likes to run.', $pet);
$pet ='cat';
echo $action;

So, just stick with the function way :)

answered Oct 12, 2016 at 0:55

A l w a y s S u n n y

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