Description:
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Prerequisites
-----------------------

PDO::ATTR_EMULATE_PREPARES = false
setlocale(LC_ALL, 'fi_FI.UTF-8') // Or any other locale with comma separated decimals

The bug
-----------------------

In locales that use comma separated decimals (eg. 4,7) the MySQLND driver will truncate all FLOATs received from the server removing all precision, basically making them into ints

When using PDO, ATTR_EMULATE_PREPARES needs to be set to false in order for the driver to actually receive FLOATs from the server (otherwise PHP seems to convert everything to strings from the get go)

The PHP userspace never even receives the original representation of the value and thus has no time to mitigate this / no real workaround exists

For example trying to fetch a FLOAT type column from MySQL with the value being 4.9 in MySQL will result in (double) 4.0 on PHP side

The cause
-----------------------

The root cause of this bug lies in ext/mysqlnd/mysql_float_to_double.h in function mysql_float_to_double

On line 43 the function will first convert the FLOAT received from MySQL server into PHP string (to be later converted back to PHP double)

However the format identifier f in standard C sprintf is locale sensitive and thus will print a comma separated decimal value

After this the value is converted into PHP double, which does it's best to convert comma separated decimal, causing for example 4,9 as a value to be simply truncated into 4.0 (as this conversion does not support commas)

This is caused by regression in commit f2eadb93b9268bca86d3f67e8d8cf2fa2767a54d.

Before this commit there was a comment stating that localization is specifically ignored:

/* Convert to string. Ignoring localization, etc.
 * Following MySQL's rules. If precision is undefined (NOT_FIXED_DEC i.e. 31)
 * or larger than 31, the value is limited to 6 (FLT_DIG).
 */

However, the commit introduces localization to the MySQL FLOAT to string conversion and removes this comment.

Suggested fix
-----------------------

Roll back the behavior to the one it was before commit f2eadb93b9268bca86d3f67e8d8cf2fa2767a54d documented in the comment

There seems to be no way to force sprintf to print dot separated decimals only

I'll try making a patch for this

Backwards compatibility
-----------------------

The suggested fix should pose no backwards incompatibility issues

Only those with the prerequisites set will see higher precision in floats received from the MySQL server after this is fixed

Test script:
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setlocale(LC_ALL, 'fi_FI.UTF-8'); // Do note: the locale probably needs to be installed
$pdo = new PDO('mysql:host=localhost;dbname=database', 'user', 'password');
$pdo->setAttribute(\PDO::ATTR_EMULATE_PREPARES, false);
$pdo->query('CREATE TABLE test(broken FLOAT(2,1))');
$pdo->query('INSERT INTO test VALUES(4.9)');
var_dump($pdo->query('SELECT broken FROM test')->fetchColumn(0));

Expected result:
----------------
php shell code:1:
double(4.9)

Actual result:
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php shell code:1:
doub1le(4)