Arithmetic within parsed string

Question

I'm trying to do simple addition on a variable within a string, using string parsing, like so:

$a = 2;
echo("I have {$a+1} eyes");

But PHP crashes when I do this.

I also tried {++$a}, but that outputs as I have {++2} eyes.

The PHP manual page for string parsing has no example covering arithmetic within a string.

Is this something that's possible?

You can achieve this with concatenation. Are you explicitly looking to do it with variable interpolation like this? — El_Vanja, Feb 22 '21 at 14:57
You could do `echo("I have " . ($a+1) . " eyes");` for example — The fourth bird, Feb 22 '21 at 15:02
Yeah I was hoping for variable interpolation rather than concatenation, since I've read on here that interpolation is faster. Obviously if it's not an option I'll default to the concatenation/formatting options. — murchu27, Feb 22 '21 at 15:09
Don't optimize too early. Are you really having performance issues with the script that does this? If the answer is no, then don't worry about the speed of the operation. Go for code clarity. The crux is that interpolation allows variables, but not expressions. — El_Vanja, Feb 22 '21 at 15:19
Sure thing. Do you want to put that in an answer and I'll accept it? — murchu27, Feb 22 '21 at 15:25

score 2 · Accepted Answer · answered Feb 22 '21 at 23:56

It's not possible through variable interpolation. It does not accept operations as arguments. What it does accept can be read in this question.

Alternatives:

Store the result into a variable first. A bit of a redundant step, but is a way to do it with interpolation.

$a = 2;
$b = $a + 1;
echo "I have {$b} eyes";

Use concatenation. Don't forget to wrap it in parentheses, because math operators don't have precedence over concatenation.

$a = 2;
echo "I have ".($a + 1)." eyes";

Use a format printing function. Note that printf outputs the string, while sprintf returns it, so you'd have to explicitly output the latter.

$a = 2;
printf("I have %d eyes", $a + 1);
echo sprintf("I have %d eyes", $a + 1);

1 Answers1