When call RESTful API containing JSON data in the body, server needs to take its content, but getting it by parameters may have some restrictions like data length or etc. To avoid that error, the easiest way can do it will be taking it from entity body directly. file_get_contents('php://input') will help you to solve it easily.
Below example is what I tested in PHP.
Let us assume we need to call following calling method:
curl -n -X POST 'http://magic.its-newid.net/api_test.php' -H 'Token: {blabhlbah}' -H 'Accept: text/xml, application/xml, application/json' -H 'Content-Type: application/json' -d '{"channels":[{"channelId":"gentleman","channelName":"NEW K.ID","channelNumber":"511","logoUrl":"https:\/\/static.foo.com\/media\/fbfaux1c4d578f426943e0b1d94f81bfb18fce~mv2.png","logoWidth":100,"logoHeight":50,"channelGenreCode":"62","adultFlag":false,"paidFlag":false,"adFlag":true,"tmsFlag":false,"promotionFlag":true,"contact":"+821025268644","resolution":"FHD","channelType":"VOD","streamContainerType":"MPD","regionList":[{"countryCode":"GB","countryRegionCode":"all"},{"countryCode":"DE","countryRegionCode":"all"},{"countryCode":"FR","countryRegionCode":"all"},{"countryCode":"ES","countryRegionCode":"all"},{"countryCode":"IT","countryRegionCode":"all"}],"activeTimePeriodStart":"2020-07-31T00:00:00Z","activeTimePeriodEnd":"2029-07-31T00:00:00Z","visible":true,"deviceList":[{"deviceName":"LG_TV","deviceVersion":"all"}],"relatedProgramIdList":[{"programId":"5db2aae1df86dve35c177ac97b"},{"programId":"5e40c1a6se703a05000158d6df"},{"programId":"5e4b39cd5f3f57d0001e90d55"},{"programId":"5e45ec51fbfb400001e49664"},{"programId":"5dc9f3eb70d7635e200ee83d"},{"programId":"5db4ceafee0dda95bec1af4ae"},{"programId":"5e40a997f742bb00018d2657"},{"programId":"5e4202c07628b30001f987eb"},{"programId":"5e4e1d675f3f570001e93be9"},{"programId":"5e4a67c82f6eaf000119bea8"},{"programId":"5e4ca3a12f6eaf000119d9f9"},{"programId":"5e3b78f02837c1500013d0f42"},{"programId":"5e4a589b2f6eafx000119be12"},{"programId":"5e4ba3cd5fb3f570001e915d4"},{"programId":"5e4e1c2e2f6eaf000119ef42"},{"programId":"5e4e171b5fd3f570001e93a3e"},{"programId":"5e3d26c1e70305000158b539"},{"programId":"5e466614b1c387000109330c"}]},{"channelId":"newid_002","channelName":"MUKBANG","channelNumber":"12","logoUrl":"https:\/\/static.wixstatic.com\/media\/fbfe8c_4d161de9f09142788ce4059a837e7bd0~mv2.png","logoWidth":100,"logoHeight":50,"channelGenreCode":"57","adultFlag":false,"paidFlag":false,"adFlag":true,"tmsFlag":false,"promotionFlag":true,"contact":"+821025268644","resolution":"FHD","channelType":"VOD","streamContainerType":"MPD","regionList":[{"countryCode":"GB","countryRegionCode":"all"},{"countryCode":"DE","countryRegionCode":"all"},{"countryCode":"FR","countryRegionCode":"all"},{"countryCode":"ES","countryRegionCode":"all"},{"countryCode":"IT","countryRegionCode":"all"}],"activeTimePeriodStart":"2020-07-31T00:00:00Z","activeTimePeriodEnd":"2029-07-31T00:00:00Z","visible":true,"deviceList":[{"deviceName":"TV","deviceVersion":"all"}],"relatedProgramIdList":[]},{"channelId":"newid_003","channelName":"PBATour","channelNumber":"13","logoUrl":"https:\/\/static.wixstatic.com\/media\/fbfe8c_b23be6440003417e8591b7db10ece72d~mv2.png","logoWidth":100,"logoHeight":50,"channelGenreCode":"23","adultFlag":false,"paidFlag":false,"adFlag":true,"tmsFlag":false,"promotionFlag":true,"contact":"+821025268644","resolution":"FHD","channelType":"VOD","streamContainerType":"MPD","regionList":[{"countryCode":"GB","countryRegionCode":"all"},{"countryCode":"DE","countryRegionCode":"all"},{"countryCode":"FR","countryRegionCode":"all"},{"countryCode":"ES","countryRegionCode":"all"},{"countryCode":"IT","countryRegionCode":"all"}],"activeTimePeriodStart":"2020-07-31T00:00:00Z","activeTimePeriodEnd":"2029-07-31T00:00:00Z","visible":true,"deviceList":[{"deviceName":"TV","deviceVersion":"all"}],"relatedProgramIdList":[]},{"channelId":"newid_004","channelName":"MUSIC&NEW","channelNumber":"14","logoUrl":"https:\/\/static.wixstatic.com\/media\/fbfe8c_d821c776b7cc40cba4ab3e0e4594572b~mv2.png","logoWidth":100,"logoHeight":50,"channelGenreCode":"18","adultFlag":false,"paidFlag":false,"adFlag":true,"tmsFlag":false,"promotionFlag":true,"contact":"+821025268644","resolution":"FHD","channelType":"VOD","streamContainerType":"MPD","regionList":[{"countryCode":"GB","countryRegionCode":"all"},{"countryCode":"DE","countryRegionCode":"all"},{"countryCode":"FR","countryRegionCode":"all"},{"countryCode":"ES","countryRegionCode":"all"},{"countryCode":"IT","countryRegionCode":"all"}],"activeTimePeriodStart":"2020-07-31T00:00:00Z","activeTimePeriodEnd":"2029-07-31T00:00:00Z","visible":true,"deviceList":[{"deviceName":"TV","deviceVersion":"all"}],"relatedProgramIdList":[{"programId":"5e4e1c045f3f570001e93b94"},{"programId":"5e4e1bc110726e0001cd3853"},{"programId":"5e4e1c115f3f570001e93b98"},{"programId":"5e4e1c3d2f6eaf000119ef46"},{"programId":"5e4e1c4310726e0001cd385f"},{"programId":"5e4e1c2f5f3f570001e93ba0"},{"programId":"5e4ca2585f3f570001e924d9"}]}]}'
And below server example will take the JSON body:
<?php
$entityBody = file_get_contents('php://input');
echo "STDIN\n\n" . $entityBody . "\n\n";
?>