Very rare since a MySQL client library is distributed with PHP and built into PHP by default. However, it is possible to build PHP without MySQL support. Some possible fixes:
apt-get install php-mysql
rpm -Uvh php-mysql-4.2.2-1.i386.rpm
./configure –with-mysql=shared,/usr
cp modules/mysql.so /usr/local/lib/php
extension_dir=/usr/local/lib/php
extension=mysql.so

Make sure MySQL is running
prompt:> mysqlshow
+———–+
| Databases |
+———–+
| mysql |
| test |
+———–+

The simple connection

$conn = mysql_connect('localhost');
echo $conn;
?>

Output:
Resource id #28

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