The logic behind how PHP interprets the concatenation of string and numbers?

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

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

Why is the expression "2 + '6 apples'' equal to 8? p>

In short, when evaluating a string to convert it to a numeric value, PHP will check content from left to right. If the first few 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 mentioned in the other answers and fundamental to the correct analysis is the precedence of the operators. The operators used are concatenation, . , and addition, + . When checking the operators precedence table in documentation :

Noticethatbothoperatorshavethesameprecedence,sotheexpressionwillbeparsedfromlefttoright,operatortooperator.

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

Thisprocessisevidentwhenevaluatingtheopcodesgeneratedby 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 site 3v4l.org , the same output is generated in all versions tested, although only in versions higher than 7.1 that a alert is issued on the conversion of a non-numeric string .

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

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

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

First PHP concatenates the value hello plus the number 1 it generates an invalid number it is then converted to zero then it performs the count 0 + 2 and counts the result of the sum with 34 ending with the string 234 .

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

This code is the equivalent of this:

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

Relationships:

Why is the expression "2 + '6 apples'' equal to 8? p>

Mathematical operation between string and number in php

The problem is that when PHP converts strings to number it gives 0 if the string does not have digits at startup. 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 you see the + operator, it converts hello1 to number (which gives 0),
• then add that 0 to 2 (which gives 2),
• and finally concatenate 2 with the string 34 .