Arithmetic operation within string concatenation without parenthesis causes strange result

Consider the f开发者_C百科ollowing line of code:

<?php
$x = 10;
$y = 7;

echo '10 - 7 = '.$x-$y;
?>

The output of that is 3, which is the expected result of the calculation $x-$y. However, the expected output is:

10 - 7 = 3

My question therefore is, what happened to the string that I'm concatenating with the calculation? I know that in order to produce the result I expected, I need to enclose the arithmetic operation in parenthesis:

<?php
$x = 10;
$y = 7;

echo '10 - 7 = '.($x-$y);
?>

outputs

10 - 7 = 3

But since PHP does not complain about the original code, I'm left wondering what the logic behind the produced output in that case is? Where did the string go? If anyone can explain it or point me to a location in the PHP manual where it is explained, I'd be grateful.

Your string '10 - 7 = ' is being concatenated with $x. Then that is being interpreted as an int which results in 10 and then 7 is subtracted, resulting in 3.

For more explanation, try this:

echo (int) ('10 - 7 = ' . 10); // Prints "10"

More information on string to number conversion can be found at http://www.php.net/manual/en/language.types.string.php#language.types.string.conversion

If the string starts with valid numeric data, this will be the value used

In this code:

echo '10 - 7 = '.$x-$y;

The concatenation takes precedence, so what you're left with is this:

echo '10 - 7 = 10'-$y;

Because this is trying to perform integer subtraction with a string, the string is converted to an integer first, so you're left with something like this:

echo (int)'10 - 7 = 10'-$y;

The integer value of that string is 10, so the resulting arithmetic looks like this:

echo 10-$y;

Because $y is 7, and 10 - 7 = 3, the result being echoed is 3.

. and - have the same precedence, so PHP is reinterpreting '10 - 7 = 10' as a number, giving 10, and subtracting 7 gives 3.

PHP runs operations in the order defined here ; https://www.php.net/manual/en/language.operators.precedence.php

Take a look at this example ;

$session_period = 30;    
new \DateTime('now -' . $session_period+1 . ' minutes');

Beware! This will NOT give you the time 31 minutes ago. In this case PHP just interprets starting from the leftmost part of an expression so this simple-looking expression returns the wrong result;

Because ;

• 'now -' . $session_period => 'now -30'
• then PHP will cast the string to 0 and add 1 to that => 1
• and 1 . ' minutes' => '1 minutes'

That's why the expression above will give you the result of

new \DateTime('1 minutes')

To avoid this kind of confusion, use () like this ;

new \DateTime('now -' . ($session_period+1) . ' minutes');