Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://www.floating-point-gui.de/

Thank you for your interest in PHP.

And just for the record, no, the same C code definitely does not return the 
"correct numbers" as you put it:

#include <stdio.h>
#include <math.h>
int main(char *argv[], int argc) {
	double a = 1.2;
	double b = 0.4;
	double v = a/b;
	printf("v(15) = %.15f\n", v);
	printf("v(16) = %.16f\n", v);
	printf("v = %f\n", v);
	printf("floor(v) = %f\n",floor(v));
}

That, I hope you would agree, is the closest thing to the same C code.

% cc a.c -o a -lm
% ./a
v(15) = 3.000000000000000
v(16) = 2.9999999999999996
v = 3.000000
floor(v) = 2.000000

That looks remarkably like the PHP output.

Thanks for explanation, I'll modify PHP code to do some round()s or casts to float to get expected results.

I used float instead of double (which uses less decimal points) in my C code, so I got correct results.

#include <math.h>
#include <stdio.h>
int main(char **argv, int argc) {
  float a = 1.2;
  float b = 0.4;
  float v = a/b;
  printf("v(15) = %.15f\n", v);
  printf("v(16) = %.16f\n", v);
  printf("v = %f\n", v);
  printf("floor(v) = %f\n", floor(v));
}

v(15) = 3.000000000000000
v(16) = 3.0000000000000000
v = 3.000000
floor(v) = 3.000000