PHP string number concatenation messed up

Question

I got some PHP code here:

<?php
    echo 'hello ' . 1 + 2 . '34';
?>

which outputs 234,

But when I add a number 11 before "hello":

<?php
    echo '11hello ' . 1 + 2 . '34';
?>

It outputs 1334 rather than 245 (which I expected it to). Why is that?

Solution 1

That's strange...

But

<?php
    echo '11hello ' . (1 + 2) . '34';
?>

or

<?php
    echo '11hello ', 1 + 2, '34';
?>

fixes the issue.

---

UPDATE v1:

I finally managed to get the proper answer:

'hello' = 0 (contains no leading digits, so PHP assumes it is zero).

So 'hello' . 1 + 2 simplifies to 'hello1' + 2 is 2. Because there aren't any leading digits in 'hello1' it is zero too.

---

'11hello ' = 11 (contains leading digits, so PHP assumes it is eleven).

So '11hello ' . 1 + 2 simplifies to '11hello 1' + 2 as 11 + 2 is 13.

---

UPDATE v2:

From Strings:

The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits.

Solution 2

The dot operator has the same precedence as + and -, which can yield unexpected results.

That technically answers your question... if you want numbers to be treated as numbers during concatenation, just wrap them in parentheses.

<?php
    echo '11hello ' . (1 + 2) . '34';
?>

Solution 3

You have to use () in a mathematical operation:

echo 'hello ' . (1 + 2) . '34'; // output hello334
echo '11hello ' . (1 + 2) . '34'; // output 11hello334

Solution 4

If you hate putting operators in between, assign them to a variable:

$var = 1 + 2;

echo 'hello ' . $var . '34';

Solution 5

You should check the PHP type conversion table to get a better idea of what's happening behind the scenes.

View more solutions

Comments

[BlitZ]

Still, it does not explains why php ignores concat operator and typecast string to int while concating. Acting as sum, but not as concat.

[JustinHo]

Hi,man. thanks for the quick reply. 245 is what I expected it to output, but it showed 1334. I just don't understand why the precedence changed if I add number before hello string.

[BlitZ]

PHP very like to convert everything to numbers. Read here.

[JustinHo]

Cheers, this helps. I really should just stick to php manual. coz the one that I read earlier says concatenation has higher precedence than addition.... what a shame......

[JustinHo]

@CORRUPT Thanks a lot! A complete answer and easy to understand. Cheers~

[Rain]

In PHP 8 "+" and "-" now take a higher precedence. So the problem is resolved.