fix: error in variable name

[apfox]

What kind of change does this PR introduce? (pls check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe below

Description

Each time db() is called, a new connection is created.

var_dump(spl_object_id(db()->connection()));
var_dump(spl_object_id(db()->connection()));
var_dump(spl_object_id(db()->connection()));

// output
    23
    28
    23

Does this PR introduce a breaking change? (check one)

• Yes
• No

Related Issue

NA