Example code
Step 1
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Using WordPress filter to modify any page meta values
In the following example you can see how to change page layout to left.
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How and where should I add this PHP codes?
Best solution is to add custom actions and filters in Child Theme functions.php file. But also you can create your own plugin and use this codes in your plugin.
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How to find any page meta settings key?
In the below screenshot, you can see how to find hide header page meta key. First edit your page in backend then find your option and then open your browser inspect element feature to find the input name or data-depent-id attribute. Exactly like below shot you can find key. For hide header key is hide_header
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