Ok, so first off, let me say this: Use while(true), as it gives the most semantic meaning. You need to parse for (;;) as it's not something you see often.

With that said, let's analyze:

Opcodes

The code

while(true) {    break;}echo "hi!";

Compiles down to the opcodes:

0: JMPZ(true, 3)1: BRK(1, 3)2: JMP(0)3: ECHO("hi!")

So basically, it does a check if "true", and if not, jumps to the 4th opcode which is the echo opcode). Then it breaks (which is really just a static jump to the 4th opcode). Then the end of the loop would be an unconditional jump back to the original check

Compare that to:

for (;;) {    break;}echo "hi!";

Compiles down to:

0: JMPZNZ(true, 2, 4)1: JMP(0)2: BRK(1, 4)3: JMP(1)4: ECHO("hi!")

So we can immediately see that there's an extra opcode in the for(;;) version.

Opcode Definitions

JMPZ(condition, position)

This opcode jumps if the condition is false. If it is true, it does nothing but advance one opcode.

JMPZNZ(condition, pos1, pos2)

This opcode jumps to pos1 if the condition is true, and pos2 if the condition is false.

JMP(position)

This opcode always jumps to the opcode at the specified position.

BRK(level, position)

This breaks level levels to the opcode at position

ECHO(string)

Outputs the string

Are They The Same

Well, looking at the opcodes, it's clear that they are not identical. They are ==, but not ===. The while(true) loop does a conditional jump followed by code followed by an unconditional jump. The for(;;) loop does a conditional jump, followed by code, followed by an unconditional jump, followed by another unconditional jump. So it does an extra jump.

Opcache

In 5.5, the Optimizer portion of opcache will optimize static conditional jumps.

So that means the while(true) code will optimize down to:

0: BRK(1, 2)1: JMP(0)2: ECHO("hi!")

And for(;;) loop becomes:

0: BRK(1, 2)1: JMP(0)2: ECHO("hi!")

This is because the optimizer will find and optimize out jump-chains. So if you're using 5.5's built-in opcache, they will be identical...

Caution

This is a complete and utter micro-optimization to base a decision on. Use the readable one. Don't use one based on performance. The difference is there, but it's trivial.