The logic behind how PHP interprets string and number concatenation?

Question

I was doing a quiz questions for PHP certification and a question appeared that left me troubled:

What will be the output of the following code fragment?

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

In short: The output will be 234. I ran via command line with PHP 7.0 and 5.6 to confirm and the result remained the same.

I still haven’t found a logic of how PHP arrived at this result and I would like to understand. Who can explain to me thank you.

NOTE: If this is not the place indicated for this question, please comment that I will delete it.

Answer 1

The question checks your knowledge of two aspects of language: how PHP treats strings within a mathematical operation - that is, how the cast of string for a number - and the precedence of operators.

The first aspect was commented on in the other answers and is very well discussed in:

Why in PHP the expression "2 + '6 apples'" is 8?

In short, when evaluating a string To convert it to a numeric value, PHP will check the contents from left to right. If the first characters are a valid numeric value, then it is kept as a result. If it is not a valid value, the return will be 0.

var_dump((float) "1foo");    // float(1)
var_dump((float) "1.9foo");  // float(1.9)
var_dump((float) "foo3.14"); // float(0)

The second aspect, not cited in the other answers and fundamental for the correct analysis is the operators' precedence. The operators used are the concatenation, ., and the addition, +. When checking the operator precedence table in the documentation:

Note that both operators have the same precedence, so the expression will be analyzed from left to right, operator to operator.

'hello'. 1 + 2 .'34';

1. The first operator is concatenation, then the value is generated 'hello1';
2. The second operator is of addition, then the value 'hello1' is first evaluated as numerical, returning 0, being added with 2, resulting in 2;
3. The third operator is concatenation, so the value is generated '234';

This process is evident when assessing the opcodes generated by VLD:

line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
   3     0  E >   ADD                                              ~0      'hello1', 2
         1        CONCAT                                           ~1      ~0, '34'
         2        ECHO                                                     ~1
         3      > RETURN                                                   1

Note that for PHP, the first operation will occur even before the expression is evaluated; this is because both operators are constant.

According to the website 3v4l.org, the same output is generated in all tested versions, although only in versions higher than 7.1 an alert is issued on the conversion of a string nonnumerical.

To demonstrate how the precedence of operators is important in this case, simply replace the addition by division. According to the precedence table, the division has a precedence greater than the concatenation, so it will be evaluated before. See:

echo 'hello'. 1 / 2 .'34';  // Resultará: hello0.534

This is why when evaluating the expression, the division will be executed before, returning the value 0.5, and after, the two concatenations will be executed, returning hello0.534.

Answer 2

First PHP concatenates the value hello plus the number 1 this generates an invalid number so it is converted zero so make the account 0 + 2 and contact the result of the sum with 34 ending as the string 234.

echo 'hello'. 1 + 2 .'34';
  ------------^   ^--------
    1 parte         2 parte 
    valor: 0    +   valor: 2

That code is equivalent to that code:

echo ('hello'. 1) + 2 .'34';

Related:

Why in PHP the expression "2 + '6 apples'" is 8?

Mathematical operation between string and number in php

Answer 3

The problem is that when PHP converts strings to number it gives 0 if the string has no digits at the beginning. That is:

echo intval('4string'); // 4
echo intval('string4'); // 0
echo intval('string'); // 0

So what happens is:

• concatenates the first string with number gives 'hello1',
• then see the operator +, converts hello1 for number (giving 0),
• then sum this 0 to 2 (giving 2),
• and finally concatenates 2 with the string 34.