PHP removes null values from the array. array removes null values.

The array_filter function uses the callback function to filter the array. it is always thought that the callback function can be used for processing, but it is not found that there is another sentence in the manual. if there is no callback function, by default, items whose values are false are deleted.

The array_filter function uses the callback function to filter the array. it is always thought that the callback function can be used for processing, but it is not found that there is another sentence in the manual. if there is no callback function, by default, items whose values are false are deleted.

  'foo',                1 => false,                2 => -1,                3 => null,                4 => ''            );     print_r(array_filter($entry));   ?>   

The output value is:

Array   (       [0] => foo       [2] => -1   )