fix: error in variable name

[apfox]

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

• [ x] 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()));

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

• Yes
• [x ] No

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