Why is floating point subtraction in PHP different from C

C code

#include <stdio.h>

union
{
    float f;
    unsigned int u;
} myun;


int main ( void )
{
    float a,b,c;

    a= 4501.490234;
    b= 4501;

    c=a-b;

    myun.f=a; printf("0x%08X %f\n",myun.u,myun.f);
    myun.f=b; printf("0x%08X %f\n",myun.u,myun.f);
    myun.f=c; printf("0x%08X %f\n",myun.u,myun.f);

    return(0);
}

      

        
        
        
      

    

Result:

0x458CABEC 4501.490234
0x458CA800 4501.000000
0x3EFB0000 0.490234

      

        
        
        
      

    

My PHP code:

<?php
$a = 4501.490234;
$b = 4501;
$c = $a - $b;
echo bin2hex(pack("f", $a))."\n";
echo bin2hex(pack("f", $b))."\n";
echo bin2hex(pack("f", $c))."\n";

$r = unpack('ffloat',"\x00\x00\xFB\x3E");
echo $r['float']."\n";

      

        
        
        
      

    

And the result:

ecab8c45
00a88c45
f3fffa3e
0.490234375

      

        
        
        
      

    

So there are two problems.
1. Different results of subtraction: f3fffa3e and 0x3EFB0000
2. Even the same 0x3EFB0000 has different representations: 0.490234375 and 0.490234