Solve high CPU problems caused by Mysql database

Server Environment Liunx AS4 + PHP5 + Mysql5 + Apache 2
When the TOP command is used to query the system performance, it is found that the CPU often reaches 100%.
I thought it was a DDOS attack ...... Firewall installed (ineffective)
Again, I started to find out if it was a system problem from the liunx system (and it didn't work)
I occasionally found an article on the Internet, and someone encountered such a problem, but did not point out the specific solution,
But I was reminded that either a system problem or a program problem!
This time, I began to filter PHP code again. Occasionally, I found that many of my brother's code is only a "regular" connection, and I didn't Close it!
I installed a database to close all the pages, and found that the CPU has been maintained at 1% ~ Between 10% (normal)
This reminds everyone not to be a bit lazy when writing PHP programs, resulting in system overflow such a low-level error!
Let's write it down here. I spent almost one afternoon on a small problem! It's also a lesson!
Remember to paste some common mysql functions! Other types of databases are similar!
Mysql_affected_rows -- gets the number of records affected by the previous MySQL operation
Mysql_change_user -- change the user logged on to the active connection
Mysql_client_encoding -- returns the name of the character set.
Mysql_close -- close the MySQL connection
Mysql_connect -- open a connection to the MySQL server
Mysql_create_db -- create a MySQL database
Mysql_data_seek -- move the internal result pointer
Mysql_db_name -- get result data
Mysql_db_query -- send a MySQL Query
Mysql_drop_db -- discard (delete) a MySQL database
Mysql_errno -- returns the numeric encoding of the error message in the previous MySQL operation
Mysql_error -- returns the text error message generated by the previous MySQL operation.
Mysql_escape_string -- escape a string for mysql_query
Mysql_fetch_array -- get a row from the result set as an associated array, or a number array, or both
Mysql_fetch_assoc -- get a row from the result set as the associated array
Mysql_fetch_field -- obtains column information from the result set and returns it as an object
Mysql_fetch_lengths -- get the length of each output in the result set
Mysql_fetch_object -- get a row from the result set as an object
Mysql_fetch_row -- obtains a row from the result set as an enumerated Array
Mysql_field_flags -- obtains the flag associated with the specified field from the result
Mysql_field_len -- returns the length of the specified field
Mysql_field_name -- obtains the field name of the specified field in the result.
Mysql_field_seek -- set the pointer in the result set to the specified field offset.
Mysql_field_table -- get the name of the table where the specified field is located
Mysql_field_type -- specify the field type in the result set
Mysql_free_result -- release result memory
Mysql_get_client_info -- Obtain MySQL client information
Mysql_get_host_info -- Obtain MySQL host information
Mysql_get_proto_info -- Obtain MySQL protocol information
Mysql_get_server_info -- Obtain MySQL Server Information
Mysql_info -- get the information of the last Query
Mysql_insert_id -- get the ID generated by the previous INSERT operation
Mysql_list_dbs -- list all databases on the MySQL server
Mysql_list_fields -- list fields in MySQL results
Mysql_list_processes -- list MySQL Processes
Mysql_list_tables -- list tables in the MySQL database
Mysql_num_fields -- obtain the number of fields in the result set.
Mysql_num_rows -- get the number of rows in the result set
Mysql_pconnect -- open a persistent connection to the MySQL server
Mysql_ping -- Ping a server connection. If there is no connection, reconnect.
Mysql_query -- send a MySQL Query
Mysql_real_escape_string -- escape special characters in strings used in SQL statements and take into account the connected Current Character Set
Mysql_result -- get result data
Mysql_select_db -- select MySQL database
Mysql_stat -- get the current system status
Mysql_tablename -- get the table name
Mysql_thread_id -- returns the ID of the current thread
Mysql_unbuffered_query -- a row that sends an SQL query to MySQL and does not obtain or cache the result.