Why does the print_r print before the echo?

Question

I got the following array:

$array = array(10,20,30);
print_r($array);

Exit:

Array
(
    [0] => 10
    [1] => 20
    [2] => 30
)

---

If I print with echo before the print_r:

echo 'primeiro: ';
print_r($array);

Exit:

primeiro: Array
(
    [0] => 10
    [1] => 20
    [2] => 30
)

---

If I print concatenating, the print_r is printed before:

echo 'primeiro: ' . print_r($array);

Exit:

Array
(
    [0] => 10
    [1] => 20
    [2] => 30
)
primeiro: 1

Still, print that one 1 in front of the primeiro.

---

Why does this happen? What is this 1 ?

Answer 1

Because print_r returns earlier:

print_r returns a value. Because of this, echo awaits all operations within its parameters before executing to be able to print the value that print_r returned.

This would be analogous to doing a mathematical operation within the echo: first the operation will be solved in, then demonstrate the value.

Because it returns 1:

According to the documentation of print_r:

When the Return parameter is TRUE, this function will return a string. Otherwise, the value returned will be TRUE.

As the parameter return was not defined, and it is FALSE by default, the function print_r is returning TRUE. This is converted to "1" within the echo.