Short answer. Almost never on its own. Conspiracy exists once two or more persons agree on committing a felony and decide to commit it — but the bare agreement is punishable only where a specific law provides a penalty for conspiring to commit that particular felony. Otherwise it is punished only through the crime actually carried out.
What the law says
A conspiracy exists when two or more persons come to an agreement concerning the commission of a felony and decide to commit it.
Revised Penal Code, Article 8 — Conspiracy And Proposal. Read the full provision →
The general rule is that talk is not a crime
The Revised Penal Code opens this article with the limit, not the definition: conspiracy and proposal to commit a felony are punishable only in the cases in which the law specially provides a penalty therefor. That ordering is deliberate. Philippine criminal law punishes acts, not intentions, so two people planning a robbery that never happens are generally not liable for a crime called conspiracy. Only where the legislature has singled out a felony as dangerous enough to punish at the planning stage — treason, rebellion, coup d'état, sedition, and certain offences under special laws — does the agreement itself become a separate punishable act.
The two elements
Where conspiracy is charged, both elements must be present. There must be an agreement between two or more persons concerning the commission of a felony, and they must decide to commit it. Idle discussion, exploring an idea, or one person's private plan does not qualify. The decision element is what separates conversation from conspiracy. Because people who plan crimes rarely sign anything, the agreement is almost always proven by circumstances — coordinated movements, assigned roles, a common design visible in how the participants acted before, during and after the event.
Proposal is different
The same article defines proposal: it exists when the person who has decided to commit a felony proposes its execution to some other person or persons. Two differences matter. A proposal needs only one decided offender, not an agreement between several; and the moment the other person accepts, you no longer have a proposal but a conspiracy. Proposal is likewise punishable only where a specific provision says so, and the list is even narrower. A rejected suggestion to commit an ordinary felony is therefore generally not a crime under this article.
Where conspiracy really bites
The reason conspiracy dominates so many criminal trials is not this article's penalty clause but its effect on liability once the crime is carried out. Where a conspiracy is proven and the felony is committed, the participants are treated as acting together, so a person who never struck a blow can answer for what the group did. That is why an accused's exposure can be far wider than his own physical acts, and why the defence usually contests the existence of the agreement rather than the acts themselves. Anyone facing an information that alleges conspiracy should get counsel involved before giving any statement — explanations offered informally to investigators are frequently read as proof of the common design.