Your loop is not running four times; it's running five times.

$i<4 means the loop will terminate when $i reaches four - it terminates before that execution happens. $i<=4 means "continue looping so long as $i is less than or equal to four"

So, let's work through the examples: for($i=0; $i<=4; $i++)

• Begins, sets $i to value of 0
• Loop: is $i less than or equal to 4? $i=0, so yes. $age += 2. ($age now equals 26).
• End of first loop: $i++ ($i now equals 1).
• Loop: is $i less than or equal to 4? $i = 1, so yes. $age += 2; ($age now equals 28).
• End of second loop: $i++ ($i now equals 2).
• Loop; is $i less than or equal to 4? $i = 2, so yes. $age += 2; ($age now equals 30).
• End of third loop: $i++ ($i now equals 3).
• Loop; is $i less than or equal to 4? $i = 3, so yes. $age += 2; ($age now equals 32).
• End of fourth loop: $i++ ($i now equals 4).
• Loop; is $i less than or equal to 4? $i = 4, so yes -- $i is equal to four, as specified by <=. $age += 2; ($age now equals 36).
• End of fifth loop: $i++ ($i now equals 5).
• Loop; is $i less than or equal to 4? $i = 5, so no. Loop terminates.

Final result: $age = 36

The number of elements between 0 and positive N is N+1.

• You loop through 0 1 2 3 4.
• Those are 5 iterations.
• 24 + (2*5) = 34.

Well that's because the loop starts from 0, so it runs 5 times (0,1,2,3,4).

Loop 1 (when value is 0): 24 + 2 = 26;
Loop 2 (when value is 1): 26 + 2 = 28; 
Loop 3 (when value is 2): 28 + 2 = 30; 
Loop 4 (when value is 3): 30 + 2 = 32; 
Loop 5 (when value is 4): 32 + 2 = 34;

you can either start with 1 as:

for($i=1; $i<=4; $i++)

or make it < 4 instead of <= 4, as:

for($i=0; $i<4; $i++)