关于如何执行/保存的变量的基本PHP说明

I have the follow:

$sum = 10 + 10;

is the above line executed saved to $sum as 20 to use it from now on or if everytime i echo $sum, it will run the 10+10?

It only calculates it once to set the value of $sum.

So from then on out $sum is equal to 20.

So the 10+10 is only calculated the first time.

During the lifetime of the script the value of 10 + 10 will be assigned to the $sum variable - no further 10 + 10 will be calculated when using $sum.

PHP is not inherently lazy nor it has lazy primitives, so value assignation is executed immediately. To simulate some sort of lazy functionality you can declare a function:

funciton sumTen() {
  return 10 + 10;
}

sumTen() // will calculate the value every time.

$sum = 0;

echo $sum; // will print 0

$sum = 10 + 10;

echo $sum; // will print 20

$sum = 5;

echo $sum; // will print 5