Problems with PHP storage database efficiency

Question

Program 1:

for($i=0;$<100;$){

Connect to the database;

Execute select in advance by a value;

Disconnect from the database;

Connect to the database;

Execute updata once to store data;

Disconnect from the database;

}

The program execution takes about 1.5-2 seconds

Program 2:

Connect to the database;

for($i=0;$i<100;$ i){

Execute select once;

Execute update once;

}

Disconnect the database connection;

Execution time-consuming 0.05-0.09 seconds

The key is that my storage database program is an interface document. The program will be executed only when someone else submits a post. Thousands of users will submit posts every second. How can I create a pdo? How about permanently connecting to improve storage efficiency? The program ends after returning the result to the post submitter, and the session or pdo connection is automatically closed.