Under Article 725 of the Civil Code of the Philippines, donation is defined as:

"An act of liberality whereby a person disposes gratuitously of a thing or right in favor of another, who accepts it."

The essential elements of a donation are:

The reduction of the donor's patrimony (the donor must give something of value).
The increase in the donee's patrimony (the donee must receive and accept the gift).
The intent of liberality (an animus donandi or intention to donate must be present).

Donations may be classified as follows:

Inter Vivos:
- A donation made during the donor's lifetime.
- Governed by the rules on donations under the Civil Code.
- Takes effect immediately upon acceptance.
- Irrevocable unless explicitly provided by law (e.g., conditions or grounds for revocation).
Mortis Causa:
- A donation that takes effect upon the donor’s death.
- Governed by the rules on testamentary succession.
- Must comply with the formalities of a will.
- Revocable during the donor's lifetime.

Movable Property (Article 748):
- If the value is less than PHP 5,000:
  - May be made orally or in writing.
  - Delivery is essential for validity.
- If the value is PHP 5,000 or more:
  - Must be in writing.
  - Acceptance must also be in writing.
Immovable Property (Article 749):
- Must be made in a public document (notarized).
- Acceptance must also be made in a public document or in a separate public document.
- The donee must be notified of the acceptance.

Donor’s Capacity to Donate:
- The donor must have the capacity to contract and dispose of his/her property.
- Minors and incapacitated individuals cannot donate.
Donee’s Capacity to Accept:
- The donee must have the capacity to contract.
- The donation must not be contrary to law, morals, good customs, or public policy.
Acceptance by the Donee:
- Express or implied.
- Acceptance is essential to perfect the donation.
- Must be communicated to the donor.

Excessive Donations:
- Under Article 750, the donation cannot impair the donor's legitimate family obligations (e.g., the legitime of compulsory heirs).
Persons Disqualified from Being Donees (Article 739):
- Those guilty of adultery or concubinage with the donor.
- Persons found guilty of a crime against the donor or his/her family.
- Public officers or their spouses/relatives within the fourth degree, in connection with donations made due to their official duties.
Prohibited Donations (Article 1027 and others):
- Donations between spouses during the marriage are void, except in moderate gifts on occasions of family rejoicing.

Grounds for Revocation:
- Non-compliance with conditions (Article 764).
- Ingratitude of the donee (Article 765), which includes:
  - Committing a serious offense against the donor.
  - Impugning the donor’s legitimate family.
  - Refusing to support the donor in case of need.
Effects of Revocation:
- The donated property must be returned to the donor or his/her heirs.
- Alienations or encumbrances made by the donee in good faith are respected.

Collation (Article 1061):
- Donations inter vivos made by an heir are brought into the hereditary mass to compute the legitime of compulsory heirs.
Action for Reduction:
- Compulsory heirs may file an action for reduction if the donation exceeds the free portion of the donor’s estate.

Donor’s Tax (National Internal Revenue Code):
- Donations inter vivos are subject to donor’s tax.
- The donor must file a donor's tax return within 30 days after the donation.
- Exemptions include donations to accredited charities, government entities for public use, etc.
Documentary Stamp Tax:
- Certain donations are subject to documentary stamp tax, depending on the nature of the property donated.

Strict Construction of Donations:
- Donations are strictly construed against the donor, as they are acts of liberality that reduce the donor’s patrimony.
- Conditions and limitations imposed by the donor are to be interpreted in favor of the donor.
Proof of Intent:
- Courts evaluate the donor’s intent (animus donandi) to determine the validity of a donation, especially in contested cases.
Compliance with Formalities:
- Failure to comply with legal formalities results in the nullity of the donation, particularly for immovable property.

By adhering strictly to these principles, the validity, enforceability, and fairness of donations are maintained in Philippine civil law.