We have not been able to reproduce this segfault.  Can you please provide the PHP script that you are reproducing this with including the table and table contents you are using.  Here is our script that we are trying to reproduce it with:

<?php

$dbh = new PDO("informix:...", "...", "...");

if ($dbh) print "Connection Established!\n\n";

try {
    $sth = $dbh->prepare("select * from systables where tabtype = ?");
    $val = 'T';
    $sth->bindParam(1, $val, PDO::PARAM_STR);

    if ($sth->execute()) {
        print("<pre>\n");
        while ($row = $sth->fetch()) {
            print_r($row);
        }
        print("</pre>\n");
    }
    else {
        print("Flunked.\n");
    }
    $dbh = null;
} catch (PDOException $e) {
    print "Error!: " . $e->getMessage() . "<br/>";
    die();
}

?>

I took your sample code as is (well, connection string 
differing of course) and got the following results:

[paul@host paul]$ php pdo_crash.php 
Connection Established!

Segmentation fault
[paul@host paul]$ 



Installing shared extensions:     /var/tmp/pear-build-root/
install-PDO_INFORMIX-1.0.1/usr/local/php-5.2.0/lib/php/
extensions/no-debug-non-zts-20060613/
running: find "/var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1" -ls
1112040    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-PDO_INFORMIX-1.0.1
1911748    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr
1112065    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local
1112066    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local/php-5.2.0
1600831    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local/php-5.2.0/lib
1601602    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local/php-5.2.0/lib/php
1601603    4 drwxr-xr-x   3 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local/php-5.2.0/lib/php/extensions
1601604    4 drwxr-xr-x   2 root     root         4096 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local/php-5.2.0/lib/php/extensions/
no-debug-non-zts-20060613
1601605  172 -rwxr-xr-x   1 root     root       170303 Nov 
14 14:02 /var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1/usr/local/php-5.2.0/lib/php/extensions/
no-debug-non-zts-20060613/pdo_informix.so

Build process completed successfully
Installing '/var/tmp/pear-build-root/install-
PDO_INFORMIX-1.0.1//usr/local/php-5.2.0/lib/php/extensions/
no-debug-non-zts-20060613/pdo_informix.so'
install ok: channel://pear.php.net/PDO_INFORMIX-1.0.1
[root@host lib]# 



(gdb) bt
#0  stmt_parameter_pre_execute (stmt=0x4050b360, 
curr=0x4050b5d0)
    at /tmp/pear/cache/PDO_INFORMIX-1.0.1/
informix_statement.c:518
#1  0x0814ee7b in dispatch_param_event (stmt=0x4050b360, 
    event_type=PDO_PARAM_EVT_EXEC_PRE)
    at /home/paul/build/php-5.2.0/ext/pdo/pdo_stmt.c:149
#2  0x0814f62c in zim_PDOStatement_execute (ht=0, 
return_value=0x4050aac8, 
    return_value_ptr=0x0, this_ptr=0x4050aa70, 
return_value_used=1)
    at /home/paul/build/php-5.2.0/ext/pdo/pdo_stmt.c:453
#3  0x0828f216 in zend_do_fcall_common_helper_SPEC 
(execute_data=0xbfffd340)
    at /home/paul/build/php-5.2.0/Zend/zend_vm_execute.h:200
#4  0x0828eae1 in execute (op_array=0x40509768)
    at /home/paul/build/php-5.2.0/Zend/zend_vm_execute.h:92
#5  0x082797f3 in zend_execute_scripts (type=8, retval=0x0, 
file_count=3)
    at /home/paul/build/php-5.2.0/Zend/zend.c:1097
#6  0x08246f48 in php_execute_script 
(primary_file=0xbffff740)
    at /home/paul/build/php-5.2.0/main/main.c:1758
#7  0x082d2783 in main (argc=2, argv=0xbffff7d4)
    at /home/paul/build/php-5.2.0/sapi/cli/php_cli.c:1108
#8  0x420158d4 in __libc_start_main () from /lib/i686/
libc.so.6
(gdb) print curr->parameter
$1 = (struct _zval_struct *) 0x4050a9d8
(gdb) print param_res
$2 = (struct _param_node *) 0x0
(gdb)

This bug has been fixed in CVS.

In case this was a documentation problem, the fix will show up at the
end of next Sunday (CET) on pecl.php.net.

In case this was a pecl.php.net website problem, the change will show
up on the website in short time.
 
Thank you for the report, and for helping us make PECL better.

This bug has been fixed in CVS Head.