Please try using this CVS snapshot:

  http://snaps.php.net/php5-latest.tar.gz

And this time: TRY the snapshot for real. And provide the backtrace using that snapshot if the crash still happens.

I had to modify my source a bit to get any output.  For some reason PHP was exiting normally despite stopping half way through the script.  Here's the modified source:

<?php
error_reporting(E_STRICT | E_ALL);
$db_user = "db_user";
$db_password =  "db_pass";
$db_name = "db_name";
$db_port = 5432;

$connect_string =  "user=$db_user " .
"password=$db_password " .
"port=$db_port ".
"dbname=$db_name";

$db_link = pg_connect($connect_string);

$query = 'select 1 as one';
$result = pg_exec($db_link, $query);
$i = (float) floor(4/5);
echo("PRE"); flush();
$data =  pg_fetch_array($result, $i); // i think it gets corrupted here
echo('i:'.$i); flush();

$data = pg_fetch_array($result, ($i));
$data = pg_fetch_array($result, ($i));
echo("z\n");
?>

And the backtrace:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1213273280 (LWP 17172)]
0xb7d455e7 in memcpy () from /lib/tls/libc.so.6

(gdb) bt
#0  0xb7d455e7 in memcpy () from /lib/tls/libc.so.6
#1  0x08289e15 in concat_function (result=0xbfffc814, op1=0x8493d44,
    op2=0xbfffc540) at /usr/src/php5-200510021630/Zend/zend_operators.c:1180
#2  0x082c2bc8 in ZEND_CONCAT_SPEC_CONST_CV_HANDLER (execute_data=0xbfffc8a0)
    at zend_vm_execute.h:3502
#3  0x082bb177 in execute (op_array=0x848ce2c) at zend_vm_execute.h:88
#4  0x0828ea39 in zend_execute_scripts (type=8, retval=0x0, file_count=3)
    at /usr/src/php5-200510021630/Zend/zend.c:1087
#5  0x08245090 in php_execute_script (primary_file=0xbfffed20)
    at /usr/src/php5-200510021630/main/main.c:1677
#6  0x0830943c in main (argc=4, argv=0xbfffedf4)
    at /usr/src/php5-200510021630/sapi/cli/php_cli.c:1039

Here's some extra information which I hope will help:

(gdb) frame 1
#1  0x08289e15 in concat_function (result=0xbfffc814, op1=0x8493d44,
    op2=0xbfffc540) at /usr/src/php5-200510021630/Zend/zend_operators.c:1180
1180                    memcpy(result->value.str.val+op1->value.str.len, op2->value.str.val, op2->value.str.len);
(gdb) print result->value.str
$1 = {val = 0x5d54802c "i:", len = 1515870812}
(gdb) print op1->value.str.len
$2 = 2
(gdb) print op2->value.str.val
$3 = 0x0
(gdb) print op2->value.str.len
$4 = 1515870810
(gdb)    

(gdb) frame 2
#2  0x082c2bc8 in ZEND_CONCAT_SPEC_CONST_CV_HANDLER (execute_data=0xbfffcd20)
    at zend_vm_execute.h:3502
3502            concat_function(&EX_T(opline->result.u.var).tmp_var,
(gdb) print *opline
$74 = {handler = 0x82c2b78 <ZEND_CONCAT_SPEC_CONST_CV_HANDLER>, result = {
    op_type = 2, u = {constant = {value = {lval = 580,
          dval = 2.86558074587923e-321, str = {
            val = 0x244 <Address 0x244 out of bounds>, len = 0}, ht = 0x244,
          obj = {handle = 580, handlers = 0x0}}, refcount = 0, type = 0 '\0',
        is_ref = 0 '\0'}, var = 580, opline_num = 580, op_array = 0x244,
      jmp_addr = 0x244, EA = {var = 580, type = 0}}}, op1 = {op_type = 1,
    u = {constant = {value = {lval = 139013284,
          dval = 4.3126732698705173e-314, str = {val = 0x8492ca4 "i:",
            len = 2}, ht = 0x8492ca4, obj = {handle = 139013284,
            handlers = 0x2}}, refcount = 2, type = 6 '\006',
        is_ref = 1 '\001'}, var = 139013284, opline_num = 139013284,
      op_array = 0x8492ca4, jmp_addr = 0x8492ca4, EA = {var = 139013284,
        type = 2}}}, op2 = {op_type = 16, u = {constant = {value = {lval = 8,
          dval = 3.3951932659396882e-313, str = {
            val = 0x8 <Address 0x8 out of bounds>, len = 16}, ht = 0x8,
          obj = {handle = 8, handlers = 0x10}}, refcount = 1,
        type = 6 '\006', is_ref = 0 '\0'}, var = 8, opline_num = 8,
      op_array = 0x8, jmp_addr = 0x8, EA = {var = 8, type = 16}}},
  extended_value = 0, lineno = 20, opcode = 8 '\b'}

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.